ye chaiva saatvikaa bhaavaa rajasaastaamasaashcha ye |
matta evati taanviddhi na tvaham teshu te mayi || 12 ||

And indeed, all saatvik qualities, and all raajasic and taamsic qualities, know those to be only from me. Know that I am not in them, but they are in me.
ye : all
cha : and
eva : indeed
saatvikaaha : saatvik
bhaavaaha : qualities
rajasaaha : raajasic
taamasaaha : taamasic
cha : and
ye : all
mattaha : from me
eva: only
iti : this
taan : those
viddhi : know
na : not
tu : but
aham : I am
teshu : in them
te : they
mayi : are in me
Shri Krishna began this chapter by indicating that Ishvara is the essence of everything. He also provided a short list containing tangible examples of his glories, which he concluded in the previous shloka. In this shloka, he summarizes this point by saying that the three fundamental aspects of prakriti or nature arise from Ishvara, but are subservient to Ishvara.
Sattva, rajas and tamas are the three aspects of prakriti or nature. A more detailed explanation of these three gunaas or aspects is provided in the 18th chapter. At the internal level, these aspects are the moods of our mind. A taamasic mind is dull, a raajasic mind is active and a saatvic mind is serene. Moreover, a saatvic object or thought is tied to knowledge, a raajasic object or thought is tied to activity and a taamasic thought or object is tied to ignorance, sloth or procrastination.
We see all these three aspects in our personality. The body is taamasic. It likes to be in stasis, it does not like to move. A lot of energy is required to move our bodies. Our limbs and our praanas or physiological processes are raajasic because they embody action and movement. Our mind and intellect are saatvic because they embody knowledge. Later in the Gita, Shri Krishna will classify everything, including desire and faith, into these three classications.
Now, all three aspects of prakriti cannot exist by themselves. Nothing can exist without a base or a support. Shri Krishna says that it is Ishvara who provides existence to these three aspects of prakriti. However, he qualifies that statement by pointing out that they are in Ishvara, but Ishvara is not in them. In other words, Ishvara can exist without the gunaas, but the gunaas cannot exist without Ishvara because they need his support.
What is the implication of this statement? Every object, person or situation that we encounter in this universe is comprised of a permutation of the three gunaas. All three gunaas are supported by Ishvara. Therefore, Ishvara is everywhere, behind every object, person and situation in this universe.
So then, what prevents us from knowing Ishvara? Shri Krishna covers this topic next.