A multi-user dungeon (MUD) is like a big game you can play with other people online. The game is set in a made-up world with different lands and creatures for you and your friends to explore.
Imagine it like this: you're playing pretend with your friends, but instead of playing in a physical place, like a backyard or a bedroom, you're playing in a virtual place on the computer.
You and your friends can create characters, each with their own special abilities and personalities. Then, you can go on quests and adventures together, battling monsters and solving puzzles.
The "dungeon" part of the name comes from the idea that many of these virtual worlds are set up like underground labyrinths or castles, filled with dangerous creatures and secret treasures to be discovered.
So, in short, a multi-user dungeon is like a big online game where you can play with other people and explore a virtual world together.