Boy, is this question taking me back! I haven't had to play directly with the stack in years!
The stack referred to above is simply the stack of numbers that various mathematical functions are working with. Think of the values in the stack like the stacks of clean plates at the beginning of a cafeteria line. Only the top number is accessible. Push shoves a new number onto the stack, and pop takes one off.
By the way, you will get better quality answers if you write better quality questions. Grammar, syntax, and spelling count!