Birds of a feather flock together means that people who are similar to each other often spend time together. Example: "Those guys work at the same company and go to the same bars. They do everything together." Reply: "Birds of a feather flock together."

We know that birds are of the same type when they have the same feathers; they are of a feather. Birds flock when they join together in groups with other birds. Just as birds "of a feather" often flock with other birds of the same "feather", so do people who are like each other spend time together. Example: "Are Timmy and his friends going to that same bar again?" Reply: "Sure they are. Birds of a feather flock together."

People ("birds") who are similar to each other ("of a feather") often spend time ("flock") with each other ("together"). Example: "Look. The volleyball players are eating together at the same table again, as always." Reply: "Birds of a feather flock together."