Hanging pieces or “loose pieces” refers to pieces that are unprotected. Hanging pieces are not protected by any of your own pawns or pieces. If the other player attacks them, or finds a way to fork them, they are lost pieces. If you see a loose piece on your opponent’s side, look for a tactic to win it. And you should try to avoid having hanging pieces, even if there’s no obvious tactical loss, because it’s just a good safety measure to protect all your own pieces properly.