What does 'return from subroutine' mean?

Question

I'm trying to build my first ever CHIP-8 emulator from scratch using C. While writing necessary code for the instructions, I came across this opcode:

00EE - RET
Return from a subroutine.

The interpreter sets the program counter to the address at the top of the stack, then subtracts 1 from the stack pointer.

(http://devernay.free.fr/hacks/chip8/C8TECH10.HTM)

I know that a subroutine is basically a function, but what does it mean to 'return' from a subroutine? And what is happening to the program counter, stack, and the stack pointer respectively?

(One additional question): If I created an array that can hold 16 values to represent the stack, will the 'top of the stack' be STACK[0] or STACK[15]? And where should my stack pointer be?

Answer 1

To return from a subroutine is to return code execution to the point it was at before the subroutine was called.

Therefore, given that calling a subroutine pushes the current address PC+2 (+2 to jump past the call instruction) onto the stack. Returning from a subroutine will return execution to the address that was pushed to the stack by popping the address from the stack. (e.g. pc=stack[sp]; sp-=2;)

As for the additional question, it really depends on whether you define your stack as being ascending or descending. For the CHIP-8 the choice is not specified.