What does it mean that birds of a feather flock together?

People meet in love, and things meet in the same kind.

It means that similar things often get together, like-minded people get together, and vice versa, which is a general term for suitable and like-minded friends. Only people with the same energy can recognize each other, only people with the same energy can appreciate each other and only people with the same energy can become friends.

What kind of good friends you want, what kind of people you have to live with. Source: The Warring States Policy Qi Ce III.

Zhu sees red, ink sees black. Metaphorically, being close to good people can make people better, while being close to bad people can make people worse.