This verse explains that the Self is the essence and the ruling factor behind all phenomenal objects and happenings in the universe.
I gave heat: Just as electricity conditioned through various equipments expresses itself as heat in a heater, light in a bulb and cold in a fridge, the Self, the Existence itself, identifying with the phenomenon called the sun becomes the source of heat for the entire universe.

I withhold and send forth rain: The sun influences the climatic conditions of the universe thereby causing rainfall or drought.

I am immortality and also death: Self being the spark of life, It gives realistic experiences of existence in the world including to the phenomenon called death. Once it is realized that the Self is Immutable and Eternal, the state of immortality is reached. Change is what is called death and therefore the Self is the illuminator of change, Itself ever changeless.

I am the Being and Non-Being : Sat (existence) and Asat (non-existence) are the terms used in Vedanta to indicate cause and effect, the manifest and unmanifest. The Self being that Illuminating factor without which neither manifest nor unmanifest can be experienced, the Self is conceived of as the Essence in both the manifest and unmanifest. He is being when manifested and non-being when the world is unmanifested.

The main idea is that the devotees regard the Lord in various ways and follow different methods of worship. The Supreme Lord grants our prayers in whatever form we worship Him.
Concepts and Issues
Observing that Arjuna was endowed with faith , free from fault finding nature and qualified to receive the sacred divine knowledge Sri Krishna says that He will disclose the Royal Knowledge and Royal Secret which will make him realize The Lord in him by direct experience. This knowledge is imperishable, knowing which one will become free from the evil of worldly existence. The Lord says that due to lack of faith in this knowledge, in its manifest and unmanifest aspects, people fail to realize Him and suffer in consequence.

Sri Krishna says that all beings are pervaded by Him and dwell in His unmanifest form like the wind moving everywhere but always resting in space. Although The Lord is the creator, destroyer and sustainer of the world, He stands as a witness indifferent and unattached to these processes.

The ignorant identify Him with nature or regard Him as a mortal during His incarnations for protecting Dharma. They do not see God in the Universe and have no knowledge of the Self which dwells within the body. They do not see the Unity in the Diversity. They fail to see the ocean beneath the waves.

They run after the transient objects and miss the eternal. They have no discrimination or right understanding. The knowers of the Truth see the Reality in externality also. They feel a oneness with all. They chant God’s name always. They worship Him with wisdom sacrifice. They see the formless one as distinct and manifold.
He explains to Arjuna His identity with the Universe. The Lord’s enumeration of His different manifestations has been tabulated below for easy reference.

Different Manifestations of Brahman

1
kratuh
Vedic ritual
19
gatih
Supreme goal

2
yajnah
Sacrifice
20
bhartaa
Supporter

3
Svadhaa
Offering to the
departed
21
prabhuh
Lord

4
aushadham
Herbage & food
grains
22
saakshee
Witness

5
mantrah
Sacred chanting
23
nivaasah
Abode

6
ajyam
Clarified butter
24
sharanam
Shelter

7
agnih
Sacred fire
25
suhrit
Well-wisher

8
hutam
Offering oblations in
fire
26
prabhavah
Origin

9
pitaa
Father
27
pralayah
Dissolution

10
maataa
Mother
28
sthaanam
Resting place

11
dhaataa
Sustainer
29
nidhaanam
Store-house

12
pitaamaha
Grandsire
30
avyayambeejam
Imperishable seed

13
vedyam
The one worth
knowing
31
tapaamy

Radiate heat as the
Sun

14
pavitram
Purifier
32
varsham
Rainfall

15
omkara
Sacred syllable OM
33
amritam
Immortality

16
rik
Rg Veda
34
mrityuh
Death

17
saama
Sama Veda
35
sat
Being

18
yajuh
Yajurveda
36
asat
Non-being

As Described in Verses 16 – 19

If one understands correctly the following verses dealing with the Divine Mystery in this section of the Gita one can be aware of the concepts of Creation and Brahman.

All things in this universe are pervaded by Me in My unmanifest aspect; all beings exist in Me but I do not exist in them. 9.4

And yet the beings do not exist in Me; behold, that is My Divine mystery. My Spirit which is the support of all beings and the source of all things does not dwell in them. 9.5

O Son of Kunti, all beings, go back to My nature at the end of a time-cycle (Kalpa); I send them forth again at the beginning of the next cycle. 9.7

Controlling My own nature, I again and again send forth all this multitude of beings, helpless under the sway of maya. 9.8

Live as the Gita Teaches You to Live

In this Chapter Sri Krishna advises Arjuna to do every thing as an offering to The Lord. All actions thus become a symbol of devotion and help remembering God.

Submission to the Divine is the essence of spiritual sadhana. The following verses tell us how to go about in that path.

But the Mahatmas (great souls) O Partha, possessing the Divine nature worship Me with undisturbed minds knowing Me as the imperishable origin of all beings. 9.13
Always glorifying Me, striving with firm resolve, bowing down to Me in devotion, always steadfast, they worship Me. 9.14

Points to Ponder
What are the knowledge and secret revealed by The Lord in this Chapter? Why they are called Royal?
What is the truth behind the world we perceive?
Enumerate the different kinds of worship.
What should be the ingredients for an offering to The Lord?
Does the Lord show any partiality in bestowing His grace on the devotees?
Short notes on:
Divine Mystery
Creation
Brahman
Attributes of the Lord
Submission to the Lord

The commentary on this verse and the rest, is avaialble for free as:
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O Arjuna, aham, I, in the form of the sun; tapami, give heat through some intense rays. Through some rays utsrjami, I pour down; varsam, rain. Having poured down, again nigrhnami, I withdraw it through some rays-for eight months. Again I pour it down in the rainy season. I am eva ca, verily; the amrtam, nectar of the gods; and mrtyuh, death of the mortals. I Myself am sat, existence-the effect which has come into bneing in relation to its cause; and its opposite, asat, nonexistence. [Nonexistence: the cause which has not become manifest as the effect possessing name and form, It cannot be admitted that the effect has absolute existence, for the Upanisad says, ‘All transformation has speech as it basis, and it is name only’ (Ch.6. 1. 4). Nor can it be said that the cause has absolute non-existence, for there is the text,’….. by what logic can the existent come verily out of nonexistence? But surely,…. all this was Existence, one without a second’ (op. cit. 6.2.2).] It is not that the Lord is Himself absolutely nonexistence; nor are effect and cause (absolutely) existence and nonexistent (respectively). Those men of Knowledge who meditate of Me while worshpping Me according to the respective forms of sacrifices mentioned above-regardomg Me as one or multifirious, etc.-, they attain Me alone according to their conceptions.