The expression is confusing because we would not normally say "birds of a feather."  We would say "the same kind of bird."  So, it means "the same kind of bird flies together in a group."   This expression is a couple of hundred years old.  I have always heard it used in a negative sense.  If someone is shocked by how a person behaves, you could say, "Well, look who he hangs out with!  Birds of a feather flock together."

Often, when speaking, people will just say the first part - "Birds of a feather" - to imply that two (or more) people act in a similar way.

It usually means that people of the same kind, personality or interests will be found in groups of like minded people :)

Thanks, Ross! Very helpful.