The Call Stack window shows the call stack for the script being debugged. The stack grows upward, with the base script at the bottom, and subsequent procedure/function/trigger calls stacked on top of one another.
The stack frame that is highlighted is the current stack frame, which displays the current point of execution and the variables that are shown in the Variables window. To view frames "below" the current stack frame, double-click a different row in the Call Stack window. The current context will change, the new execution point will be shown, and a new set of variables will be visible.