Hubris is an excess of confidence: a boxer who shouts "I'm the greatest!" even though he's about to get pummeled by a much stronger opponent is displaying a lot of hubris.
Hubris is from Greek, where it meant "excessive pride, violating the bounds set for humans" and was always punished by the gods. We no longer have the Greek gods, so in English it just refers to over-the-top self-confidence. If you call yourself the best in something, you better have the goods to back it up, since too much hubris can lead to embarrassment and humiliation. It's an age-old human failing: pride goeth before the fall.