CALL is a control transfer instruction that is used to call a particular subroutine. A subroutine is a block of instructions that need to be performed frequently.

To make sure that that the AVR knows where to come back after the execution of the subroutine, the microcontroller automatically saves the address of the instruction just below the CALL instruction on the stack. After finishing the execution of the subroutine, the RET instruction transfers control back to the caller. Hence, every subroutine has a RET instruction at the end.