MUD stands (sometimes) for multi-user dungeon… it’s essentially a text based mmorpg that’s usually light on the “massive” (the more popular MUD I played on for a while had a max concurrent user count somewhere around 120) and they tend to be very focused on role-playing and descriptions rather than mechanics - though your mileage may vary, a few are extremely focused on the mechanics and pay little attention to role-playing.
It’s quite fun and the communities are always really welcoming, I’d suggest giving it a try if it sounds interesting!