The great poet Tupac summarized his song at the end with the lines starting with “The world is behind us ...” Just like a driver who looks into his rear view mirror to see what is behind him, Tupac is looking back (remembering) at what is behind him (his bad experiences in the world). Not only is he looking back, he is moving forward (like a driver moving forward). Moving forward meaning he is more knowledgeable of the world and he knows what he needs to succeed.

The rearview means the rearview mirror in a car which allows a driver to see behind them without taking their eyes off the road. So he's watching life as if it's happening in his rearview mirror. I think it suggests he is disconnected from life and feels like life is happening somewhere else, to someone else. However since it is a song it is really up to you to decide what it means to you....