Tabletop roleplaying games, pen and paper games where you create a character and undertake adventures, have been around a very long time. The first games were based on tabletop wargames; where you pushed around little toy soldiers to play out battles (historical or fantasy). Out of this grew the idea of people controlling a single character rather than a whole army. This is the origin of 'Dungeons & Dragons' which you may have heard of.

The general scope of these games was that the players have characters (a fighter, a wizard, an elf etc) there was a 'dungeon', the dungeon has monsters, traps and treasure and your goal is to get that treasure. The game was in outsmarting the traps and tricks of the dungeon and getting out with your treasure and your life.

These kind of games have evolved a lot since their original creation, with new styles and ways of doing things. There's been many new editions published that have improved some things, made other things worse, and in some people's opinions, made things too complicated.

Alongside this, the overall trend in game design was towards richer stories; this lead to superhero style characters setting out to save the world, and everyone having complex character motivations that need to be woven into the emerging story. People felt cheated when a character they worked hard to develop and build a rich background for died suddenly, so characters tended to die less. Everybody had 'plot armour'.

About ten years ago, some folks who were feeling burned out on this style of play went back to the original 'old school' rules; where characters are mortal, motivations are simple (get as much treasure as you can without dying) and character death was frequent and sometimes unavoidable. They found that stripping away all the other complex elements that had build up in modern role playing games made the game feel fun again, more like an actual game than say, four friends brainstorming a New York times bestselling fantasy novel with an upcoming Netflix series. This was dubbed by the community as the 'Old School Revival' or 'Old School Revolution' aka the OSR.

The short version is that this is people playing Dungeons & Dragons as a game more than as a cooperative narrative building exercise.

Both styles have their fans and neither is the 'correct' way; sometimes you want to build a captivating narrative about characters struggling with inner demons while trying to slay outer demons, and sometimes you want to see how long you can survive in an unground maze filled with animated skeletons armed only with a pitchfork, a burlap sack and your burning desire for gold.

I hope that helps explain it.