All Users (or only room operators if the room is mode +t) can use this command. The topic command lets you change the topic of a channel, topics can be up to 80 characters long.
Example:
/topic #ConferenceRoom The webmaster Support Room
The topic is one of the first things people see about a room. The topic often gives some information about the room, or highlights an amusing quote that came up in the room recently. The specific use of the topic varies from room to room, but the topic should generally reflect something about the conversation going on in the room. Most rooms will limit the topic command to room operators to prevent topics that they dislike.