When playing Ultimate, one's first instinct is to run around in circles around the thrower, yelling ``I'm open! I'm open! Throw to me!'' As you might imagine, this tends to cause confusion. This is why Ultimate players have developed a structured offense.
Basically, players take turns cutting. A cut is one pass where the player tries to get open, usually running towards the disk. If the player is not immediately successful s/he runs away to give another player a chance. While players wait for their turn to cut they stand in a line called the stack. This line is downfield from the thrower (i.e., between the thrower and the endzone in which they will score) and out of the direct line where the thrower wants to throw.
Here is a stack waiting neatly in line for their turn to cut:
If you are not open or you are totally confused it is important to clear. Clearing means running somewhere the thrower can't throw (because e.g. there are people in the way), usually towards the back of the stack. When you clear you are helping your team by not getting in the way of other people who may have a better opportunity.