Chickens like to roost up high at night. When they sleep, they don't respond to much at all - hence the reason it's easier to check on one that might be having an issue while it's on the roost at night rather than try to catch it during the day. They roost because they feel safer from predators.
The terms "roost" and "roosting pole" are used interchangeably. Many people, me included, a use 2x4 with the widest, flat side up. That way they aren't trying to cling to a round pole and sleep, and their toes aren't exposed to cold air. Instead they can literally sit right on their feet to sleep, keeping them warm in winter. That said, many people use round poles for their chickens and it works just fine. But a lot of times when the question is asked, "Why won't my chickens roost on their pole", a few questions back at the poster will yield the information that they are using a closet rod or something narrow like that. Good thick branches also work very well, as long as the birds don't have to curl their feet around a uniformly narrow rod.
In addition to the 2x4 roost, I have two tree limbs out in their run...one ranges from 3 to 4 inches in diameter from one side to the other and they like having the different sizes on the same roost. The other is more uniform in size from end to end, about 4 inches in diameter.
Edited for spelling.