Stalling is when a player intentionally plays slowly in an attempt to gain an advantage due to the fact that matches in tournament Magic are timed. It often happens when a player will either win a match or at least not lose one if the current game is not completed.
It is important to note that not all slow play is stalling. The difference is the intent. Playing slowly does have its own entry in the infraction and procedure guide, but it covers situations where a player plays slowly unintentionally. Stalling only applies to when a judge believes that a player is conscious of his pace of play and deliberately doing so to run out the clock.
One final thing to point out is that it is possible to stall without actually playing slowly. Simply slowing down your normal pace of play is stalling if done in order to waste time. The important thing here is that the pace of your play has changed, not just what your speed of play is.