“Cooped” means kept indoors, often in a small space. (A “coop,” in fact, is where chickens are kept on a farm.)
“Cooped up” is very similar, and we use it to talk about people being inside too much, especially if they are inside for a long time. Being “cooped up” is not a good thing; no one wants to be stuck inside.
Winter is the worst because we have to stay cooped up inside so much.
It’s not fair to keep him cooped up in my little apartment all day.