A Gladiator is a combination of a fighter and a performer, being a character who partakes in live combat displays for the entertainment of a crowd. Gladiators are usually associated with slavery, as risking one's life for money is seen as the sort of thing usually only a crazy person would do if they had the choice.
The iconic gladiator in most peoples' minds stems from basic knowledge of the Roman Empire, in which they were hugely popular. Misconceptions about the empire fuelled by old "swords & sandals" films further fuel beliefs about gladiators, like that they were regularly expected to kill each other for the amusement of the baying hordes of spectators. In actual fact, true gladiators of the Roman Empire were probably closer to professional wrestlers today; they did fight, and they did bleed, but a lot of what they did was staged for entertainment purposes. Gladiators were big money makers, but they also cost a fortune to train up in the first place; death matches in a Colosseum were usually fought between executed criminals - it was very rare for a professional gladiator to fight to the death. Nonetheless, the image of a scarred, battered thug who kills to survive or for money as decadent nobles cheer him on remains embedded in the collective consciousness.
Roman Gladiators came in a number of archetypes, like stock characters: there was the popular Murmillo, who fought with sword and shield and wore a distinctive closed helmet, the retarius who fought with a trident and net, or the venator, who mostly fought wild animals.
In Dungeons & Dragons, gladiators first appeared as a variant Fighter class from Dark Sun in the Advanced Dungeons & Dragons game. While the Dark Sun fighter got lots of social powers revolving around leading armies and attracting followers, the gladiator stood out as the meanest bitch on the block in an edition where the fighter was already the king of combat.
This would later be remade into a character theme in 4th edition, whilst Dragon Magazine would have articles on playing gladiator-themed characters in general, for both 3rd edition and 4th edition. Returns in 5th edition as an alternate for the Entertainer Background that grants Proficiency in Acrobatics and Performance, as well as, oddly, a disguise kit and a musical instrument that can be replaced with a more traditional weapon like a trident. Pathfinder, naturally, gave the fighter an archetype that focuses on gladiatorial combat, and one sourcebook features a whole system of "Performance Combat" centered around gladiatorial contests.
In 5e the Gladiator comes back with a variant background for Entertainers. Rather then a more traditional entertainer, singing or music of the like, you can be a Gladiator, a show fighter. You can swap out your musical instrument for a inexpensive but unusual weapon, such as a trident or net, and you can use your feature to find places to fight for money.
A slight note on the Latin if your curious: Latin is a gendered language. The "Tor" in "gladiators" refers to the gender of the person. That's why a female "gladiator" would be more proplerly called a "gladiatrix", the "trix" means women. It's also why Dominatrix ends with the "trix", it's a woman "trix" dominating "Domina".