At a basic level, it's cooperative story telling. Typically, the GM controls the environment and NPCs while the players each control a character that interact with the environment and NPCs.

This cooperative story telling at the extreme ends can be either a mechanically heavy dungeon crawl focusing on fights, or plot/dialogue heavy and (usually) mechanically light. You can emphasize player agency (sandbox) or deemphasized player agency (railroad) or anything in between.

Dice are typically used to randomize success or failure of a specific action, and to procedurally generate things (though there are ttrpgs that use things besides dice or have no randomization). Randomizing success or failure of an action is typically improved by the specific abilities of the characters or NPCs. The could include physical or mental capabilities, skills, magical powers etc.

Randomization for procedural generation isn't necessary since everything can be intentionally designed, but can be fun. You can procedurally generate combat encounters, NPCs, treasure hoards, cities etc. Really anything.

As you can tell, there's a lot of ways to play ttrpgs. Even just within d&d, I've seen a lot of variety with some groups very similar to mine, some interestingly different, and some playing in a way I would find unappealing.