Imagine that you were born in a cave: you are tied to a wall and you only see shadows. Since you can't turn around to go outside, you think the real world is those shadows. Only a person who gets up and goes outside begins to understand that the world around her is much more than those shadows.
This is how Plato describes our relationship with the outside world. The Greeks claimed that our limited senses trap us in a sensory world that is transient and that only represents a part of reality. Reality, they said, has deeper, more enduring aspects, but we cannot experience them if we cannot escape the trap of our senses, so limited.
How can we get out of our cave and better understand what surrounds us?
Plato wants us to be like the philosophers, and since "philosophy begins in amazement" he invites us to experience the world from less rational, less limited and rigid perspectives.