Similar or like-minded people. A shorthand version of the full proverb "birds of a feather flock together," meaning people who have similar interests, ideas, or characteristics tend to seek out and/or associate with one another. I knew you and John would get along well, you're birds of a feather, after all.
phr. & comp. abb. people who share an interest or proclivity. Those guys are really birds of a feather. They are always together. We’re BOF and love to hike and enjoy nature.