In tf2 you receive items (weapons, hats, misc) at random but only while you are in-game. So the longer you play the more items you will get. Since some people do not always have the opportunity to play as much as they want, idling was born. You basically join the idle server and then you are free to leave your computer unattended. This records as playing time with valve and increases your chance of finding items. The server basically has to be setup not to kick afk and you typically use an idling map such as ‘achievement_idle’. Also the chances to find more items is increased the more players you have in the server.
As far as a useful link or something for this, I have yet to come across.