A stack is a term used in Ultimate Frisbee to refer to a line of offensive players. A stack gives some structure to the offense, and prevents picks and clogging. Clogging is what happens when one player is standing in the space another player needs to run into to get open and get the disc. A stack is usually in the middle of the field, with the players in single file parallel to the sidelines. The stack should probably be about 15 yards from the disc, and there should be a few yards between each player in the stack. Players make cuts out of the stack and "clear out" by running back into the stack after making a cut.

The stack developed in Ultimate Frisbee to open up space for cutters(players who are trying to get open and get thrown the disc) and avoid picks, which are illegal in Ultimate. By keeping players who are not cutting out of the area that the cutter is cutting towards, it reduces the chances that the cutter's defender will get picked, and gives the cutter more room to run in and get open.

o's are offense, x's are defense. Capital O has the disc.
|-------------------------------------------------------|
|							|
|							|
|							|
|							|
|							|
|-------------------------------------------------------|
|							|
|							|
|			   Ox				|
|							|
|							|
|							|
|							|
|							|
|	SPACE		x  o		SPACE		|
|			x  o				|
|			x  o				|
|			x  o				|
|			x  o				|
|			x  o				|
|							|
|							|
|							|
|							|
|							|
|							|
|							|
|							|
|-------------------------------------------------------|
|							|
|							|
|							|
|							|
|							|
|							|
|							|
|-------------------------------------------------------|
See the space that is open on both sides of the stack? The next step exceeds my ASCII art skills, but imagine that the offensive players can now run out of the stack into the SPACE, where they can get open and catch the disc. If everyone was just running around, there would be no space to run into and no one would be able to get open. Also, it's important to clear out once you've cut into a space, because once you're there, it's no longer space that can be cut into. So once a player has cut into the space, he needs to run back into the stack. As that player is clearing out, another player should be starting to cut into the space that the first player is now clearing out of. This cycle should continue, with one player cutting, clearing, and another player cutting into the space continuously. This should be moving the disc downfield, and the stack should move with the disc, always keeping approximately 15 yards in between the disc and the front of the stack.

That's the basic idea of the stack. There's a number of other basic concepts that are important in a basic offense, namely the dump. Also, the stack is a more abstract idea than this writeup would lead you to believe. Stacks are not just vertical lines of players in the middle of the field. There are offenses that rely on horizontal and diagonal stacks, and the stack itself needs to change and adapt to the playing conditions and the particular offense. Sometimes the stack needs to be farther back, i.e. if you were running a handler weave, or closer, if you want to make it easier to make deep cuts. If you want more information, you should look at http://www.ultimatehandbook.com, which is an excellent site with lots of good information for Ultimate players of all levels.

Side note:

Interestingly enough, now that I think about it, the stack(in Ultimate) is not a stack(a LIFO data structure). It's actually a queue, since the last person to clear into the stack should wait until the other players have cut to make another cut, which is LILO behaviour.