Q: What is a roost?
A: A roost is a perch for a chicken---what s/he sleeps on, usually a branch, pole, or board that is elevated. It also refers to the act of resting on it: roosting.
Here you can see a roost through the chicken pophole door of this coop.
Chickens don't have grasping feet the way songbirds do, for instance, so chickens prefer a flat surface. Even an elevated 2 x 4, with planed, sanded, or slightly rounded edges work nicely. Round poles or branches are okay, too, but they should be on the larger side. Whatever you do, be sure not to use metal pipe! If it gets very cold, the metal can conduct the heat away from your chicken's feet, making her colder or even leading to frostbite.