A boner is an erection. It's also a word for mistake. Either way, you probably want to hide your boner.
This is a very slangy word that is not fit for many situations. No one really wants to hear "I have a boner!" or "I made a boner!" — especially because the "erection" meaning has kind of overshadowed the "mistake" meaning. If you mess up, you're better off with a word like goof, flub, blunder, or snafu. Those words are all kind of fun, and they won't get you in any more trouble than you're already in for making a boner.
