hi david
Imagine the stack like a long cylinder with a spring inside, you shuv plates in one by one, the last plate you shuv in is the first plate you take out.
with microcontroller the stack is a memory used in the same way, push stores data in stack memory and increments stack address by one, to get your last saved byte you use pop instruction which decrements stack pointer by one.
Stack is used automatically with interrupt service routine or call instructions where current program counter is pushed (saved) on the stack and popped when the reti or return instruction executed so that program continues from address where it left off.
Regards
Mahmood
P.S HLL compilers rely heavily on the stack