Imagine a bird picking up a stone and placing it on its wing or balancing it on its beak. The question is: Would it be able to fly? Yes, birds can fly. But clearly not if something is heavy enough to weigh them down. So they know not to fly with something like a stone. Instead, they opt for things like twigs and pebbles.
Similarly, we sometimes try to fly while carrying a lot of baggage. These are things that weigh us down emotionally–mentally. This quote, by the writer Toni Morrison, reminds us to be like the birds. If we want to fly, we are going to have to give up all the things that weigh us down. Just like a bird trying to fly carrying a stone, it becomes physically impossible.