Someone who's humorless doesn't find anything funny. Your humorless uncle won't even crack a smile when you tell the joke that makes all your friends laugh hysterically.
Do you love funny movies? Then you might want to skip the one that reviewers described as humorless. This adjective is a good way to describe things (and people) that are overly serious, unsmiling, or unable to appreciate humor. The word comes from humor, which was originally a medical term meaning "bodily fluid," a fact that you might find amusing — although a humorless person definitely wouldn't.