Introduction

Consider the following scenario.
You want to write a loop that will continuously display the getkey code for the key you press, until you press enter.
The program would look something like this:

Loopz:
bcall(_GetKey)
bcall(_ClrLCDFull)
cp 5 ;If enter is pressed,
ret z ;then return
ld hl,YouPressed
bcall(_PutS)
ld h,0
ld l,a ;load a into hl
bcall(_DispHL) ;display the number
jr Loopz
YouPressed:
.db "You pressed key #",0

If you compile and run this, it will always give out the number zero. Why?
The answer is that _ClrLCDFull, among most other ROM calls, clears many of the registers. You could try loading a into another 8-bit register, but _ClrLCDFull clears ALL the registers, so that just won't work. Fortunately, there is an area of memory called the stack. The numbers stored in registers can easily be copied into this area, using a command called push. Push take a 16-bit register as an input. There is another command, called pop, which recalls the value at the top of the stack into the given register. See the edit below:

Loopz:
bcall(_GetKey) ;returns value of pressed key into 'a'
push af ;push requires a 16-bit register
bcall(_ClrLCDFull)
pop af ;pop requires a 16-bit register
cp 5 ;If enter is pressed,
ret z ;then return
ld hl,YouPressed
bcall(_PutS)
ld h,0
ld l,a ;load a into hl
bcall(_DispHL) ;display the number
jr Loopz
YouPressed:
.db "You pressed key #",0

You are not required to pop into the same register you pushed. Also, while the stack can hold quite a few variables, it is not of infinite size. Please know that, while you will likely never achieve this, continuing to push values without popping them will overflow the stack and corrupt adjacent memory areas.