The word flock refers to a group of animals, like birds or sheep, that have congregated together. The flock of pigeons at the park is so tame that they'll eat right out of your hand.
You can also use flock to mean a congregation of people, like a flock of children at the zoo, or a group of people belonging to a church. As a verb, flock means to gather together as a flock or to move together in a group. You and your friends might flock to the mall when you learn that your favorite singer will be there signing autographs.