And while we´re at it, overusing this "feature" is frowned upon, i.e. when done multiple times a day or almost every day in e.g. a sales thread in the Junkyard. Trying to keep a thread on the front page without actually adding something of value to it is rude and unfair to the others.
It's essentially a way to move the thread back to the first page. So if you're selling an item on the junk yard and you want people to see it easily and You don't want it to get lost amongst the hundreds other threads you 'bump' it back to the top.