In these lines, Sankara explains that the entire universe is seemingly real, as much as a snake is 'seemingly real' when we mistakenly look at a rope.

The snake 'manifested' on the rope for our mind because we saw it wrongly - likewise the universe 'manifested' and is 'real' for us when we see it incorrectly. This causes us samsara.

The snake appears not because of the rope - but because of our ignorance. Likewise, the world appears not because of Brahman but because of our ignorance.

And, in the concluding verse 4, Sankara says that the person who recites these three slokas everyday will achieve supreme bliss of knowledge to recognize 'real' from 'unreal'.