There are 3 main forms of communication. When other players use these commands, you will see the name of the player who did it, and what they 'said'. You can get a list of who is playing by typing 'who'. The second concept is the concept of rooms, and movement. When you are playing on a mud, you are only in one room at a time.
Rooms are how the game breaks the world into manageable chunks.
Rooms have 'exits' that lead to other rooms. These exits exist in 6 different directions, north, south, east, west, up, and down. By typing the direction you want to move, if an exit exists, you will move to the room that is in that direction. 'Look'ing in a room will show you what the room looks like, as well as which objects and characters you can see in the room with you. Which leads to the idea of objects. An object is something in the world that a character can interact with.