If you misread a party invitation and arrive in a penguin costume, only to realize that the other guests are wearing elegant gowns and tuxedos, you'll understand what it means to commit a faux pas, or an awkward social mistake.
Faux pas literally means "false step" in French, and that's a great description of what you do when you make a faux pas. It's a matter of stepping in the wrong direction, or saying exactly the wrong thing. A faux pas can offend people sometimes, but more often it's just embarrassing for everyone involved.