यथा सर्वगतं सौक्ष्म्यादाकाशं नोपलिप्यते। सर्वत्रावस्थितो देहे तथाऽऽत्मा नोपलिप्यते ॥

yathā sarva-gataṁ saukṣhmyād ākāśhaṁ nopalipyate sarvatrāvasthito dehe tathātmā nopalipyate

As the omnipresent ether is not affected, by reason of its subtlety, so seated everywhere in the body the Self is not affected.

In plain words: Krishna explains that space is everywhere but nothing stains it, because it is so fine. In the same way, the Self is present all through the body but is not affected by it.