The Stack Ultimate is a game of flow. A good offense is characterised by quick passes, one after the other, that quickly move up the field. One of the most tell-tale signs of a beginner team is the problem of 'clogging'. With fourteen players on the field at any given time, twelve of which are running in order to try and get open for the pass, things very quickly get chaotic, and disorganized.
People begin to find that it is difficult to get open because someone is always in their way.
Because picks are a violation in ultimate, you also find that occasionally you must stop so that you don't inadvertently pick an opponent. The most common strategy for reducing clogging is called 'stacking'. The idea behind the stack is simply to make room on the field. Essentially, the players line up down the field from the disc. The field directly ahead of the disc is now opened up for pass reception.