The address to which the CALL instruction takes the flow is called the subroutine
example :
2500 : mov A,B
2502: CALL 8500
2504: ADD A,B
now the instructions from 8500 till it reaches a RET ststement is the subroutine
Once RET is encountered the control goes back to address 2504 which is the main program
IN SHORT - A branching from main program to some other set of instructions(also called functions) happens during CALL
these set of instructions or functions are called subroutines
