A Stack refers to a method of placing one thing on top of another. In a cafeteria, the clean trays are placed in a stack, one after the other, and the most recently cleaned tray is on top of the stack, waiting for your use. Of course this means the tray that was cleaned longest ago is on the bottom of the stack.
As your program calls more and more "functions" the computer keeps track of this information in an "Activation Stack" (a list of all functions in the order they were called, where in the function they are, and what information is associated with them). This represents the "State" of your program. You often want to see how one function called previously has affected the "current" function. To do this, click (in the Debug window) on the STACK pull down menu. By selecting the names of the functions you "move up and down on this list" and thus see how your program "got" to where it currently is!
