Purport

In last Shloka Bhagvan Shri Krishna explained 3 important facts about the interrelation between Him the God and this universe, those three facts are
1. The entire universe is pervaded by unmanifest potency of Bhagvan Shri Krishna, the God
2. The entire universe is dependent upon him
3. Shri Krishna is not dependent upon anything or anybody in this universe.

In this Shloka, Bhagvan Shri Krishna then explained other facts explaining the interrelation between Him the God and universe, these facts are
1. [Living being are dependent upon Him the God but ] not existing in him.
2. The self or the will of Shri Krishna supports life every living being [and this is how Shri Krishna pervades entire universe]

Combining the facts from two Shlokas gives a very clear explanation of how this universe sustains and what are the correlation between God and this universe.

The unmanifest potency which has been defined as "mamātmā" = my self, my will or my own potency. The own potency of Bhagvan Shri Krishna is the cause of life in this universe. But then Shri Krishna further clarified the relation between Him the God and living being is not like the container and contained. In the next Shloka, this relation is further clarified where Shri Krishna explained that living beings exist in him the same way air exists in the sky.
Though the name of the potency is not mentioned in this Shloka, in Chapter this potency is named, as Bramhan. This bramhan is the intrinsic nature of Bhagvan Shri Krishna which sustains every living soul in this universe.

Taking this fact from here will explain the characteristics of the soul as defined in chapter 2 with much more clarity. Ch2 defines the soul as eternal and unborn. Since souls are formed of Bramhan the very own potency of Shri Krishna. Since Shri Krishna is eternal, so is Bramhan and so is the soul.