NOP (No Operation) does what it says. When the opcode is executed, the processor does nothing for two clock cycles. This is effective if you want to create a brief delay or make a gap for later additions to machine code.
Because this opcode doesn't do anything, it is a useful way of "commenting out" a section of a game. For example, there is a subroutine that decrements the player's lives, you can change the opcode and operand to NOPs, thus preventing the code from executing, effectively giving the player infinite lives.