A roost is a place where birds or bats can sleep or rest safely. Backyard chickens need a comfortable, secure roost to stay warm and rested.
You can use roost to mean the perch that birds stand on while resting, the structure that contains the perch, or even a random tree branch, in the case of a wild bird. As a verb, it means to sit or to rest: "The chickens began to roost for the evening." When someone says, "Chickens come home to roost," they mean that bad actions eventually have bad consequences. And if you "rule the roost," you're in charge.