For most sports fans the worst that could happen when a referee makes a bad call during a game is that your team would lose. However in the days of the ancient Gladiators a bad call by the “summa rudis” (referee) could mean death.
According to Carter, most gladiatorial bouts weren't necessarily meant to be played to the death, although fatal wounds were all too common. He argues that there were some fairly specific rules in place for the fights. One key aspect of the fights was submission, in which the vanquished gladiator could appeal to the patron of the fight for mercy, and if approved could then leave the arena. That's along the same basic lines as the famous "thumbs up"/"thumbs down" decision at the end of a gladiatorial fight, which is the one "rule" you almost always see in movie depictions of the fights.