Leaving your irc client connected to a server and sitting in channels even if you are not at the computer. This allows a group of users to stay in contact even if they are
not active all the time.
Many misunderstand this concept and think idling means to stare at an
inactive channel for
hours on end but it just means leaving your client connected even if you aren't there or are doing other things.
Note: things like the following would not be possible if both users were not idling.