See all articles from Syntax/Syntax ZX80 v1 n2
Subroutines are mini-programs; they execute a series of steps apart from the main program. In essence, you tell the computer, “Go do those steps and then come back here and proceed with the main program.” The ZX80 remembers where it was and returns to the following line after executing the subroutine. You can call the subroutine many times from your main program, but you need type it only once.