We have E.M Forster to thank for this term. He coined it to refer to characters who are one-dimensional, who lack emotional depth, and who don't change much over the course of the story. In other words, they're as flat as that bottle of Coke you left on your windowsill last week.
If you're already writing these guys off as boring, they're anything but. They do awesome things like help move the plot along, provide comic relief, or act as the big bad. In general, they make the other guys look round (and that's a good thing).