Used to describe things of the same kind often get together, like-minded people get together in groups, and vice versa.

From The Warring States Policy, Qi Ce III, and The Book of Changes, birds of a feather flock together. Compilation of Liu Xiang at the end of Western Han Dynasty. Extended information

English explanation:

The birds of a feather together.

It means that things of the same kind often get together, and like-minded people get together in groups. On the contrary, they are separated. Nowadays, it is more metaphorical that bad people collude with each other.

Reference: Baidu Encyclopedia-Birds of a feather flock together.