Could someone maybe explain it? A diagram would be extremely helpful, but maybe just explaining why "the top of the stack" means "the bottom of the stack"

Before the use of hardware stack pointers, the analogous imagery was a stack of plates in a spring loaded holding cart. You can only add or remove a plate from the top of the stack of plates. Adding a plate would push down the other plates, and removing a plate would pop up the next plate. But even from the beginning, the contents of the computer stack were not moved (unlike the stack of plates).