What is a mud?
In a nutshell...
A MUD (Multi User Dungeon) is a text-based adventure game played over the internet. People from all over the world can play the game at the same time and interact.

As well as other real people there are also characters known as mobiles.
What is the point?
By exploring, finding weapons, killing mobiles (game characters) and completing quests you gain points and climb up levels. The more quests you complete and levels you reach, the more abilities, strength and spells you will have in your arsenal. This makes it more fun and gains you prestige.

Above all, it should be fun. People tend to be friendly and talkative on MUDs like 7 Degrees of Freedom.