A room that's stuffy isn't well-ventilated or full of fresh air. If your classroom is stuffy, you may find it hard to concentrate. Better ask your teacher to open a window.
If your car is stuffy, it's hot and there isn't enough fresh air circulating. If your nose is stuffy, it's congested (also without enough air moving through it). When a person is stuffy, he or she is a little prim and proper, without a lot of imagination or a willingness to try new things. The very earliest meaning of this adjective, dating from about 1550, was "full of substance."