This is one of the pivotal shlokas of Chapter 6. After describing how to meditate, Krishna now describes what the accomplished yogi actually sees. And what he sees changes everything.
The yogi sees one Self present in every being — the ant, the elephant, the saint, the stranger. And at the same time, he sees every being contained within that same Self. It is not that differences vanish. Trees still look different from rivers. But behind every form, the yogi perceives the same living presence, the way a single ocean underlies a thousand different waves.
This is 'sama-darshana' — equal vision. Not indifference. Not blindness to the world's variety. It is the recognition that the same spark animates every life. When this recognition settles in, the walls between 'me' and 'everyone else' quietly dissolve.