Some birds are not meant to be caged, that's all. Their feathers are too bright, their songs too sweet and wild. So you let them go, or when you open the cage to feed them they somehow fly out past you. And the part of you that knows it was wrong to imprison them in the first place rejoices, but still, the place where you live is that much more drab and empty for their departure.

This quote is saying that some people have special qualities that make them unique. They should not be kept in a cage and should be allowed to be free. It may be hard to let them go but you will be happier in the end because they are living their own life instead of being stuck with you. The environment in which they left will be dull and empty, but you will know you did the right thing in setting them free.