In programming, a call stack is a last-in, first-out listing of procedure calls.
Real World Analogy
My wife has a question for my son, Luke, but he's off in the woods. So the question gets relayed through the whole family:
Unwinding the Stack
As each person responds, the stack shrinks:
Viewing the Call Stack
To view the call stack in VBA, go to View > Call Stack... (Ctrl + L).
Note that the call stack window is only available while the code is executing.
UPDATED [2021-11-05]: Added sample code to the VBA Call Stack screenshot; added image credit.