What does it mean when a resistor symbol in a schematic has a value of zero or "NO-POP"
NO POP means not populated (i.e. the space on the PCB is there, but there's no component). In this case you'd install nothing.

Zero-ohm resistors are used just for jumpers (so the same machinery can be used for jumpers and actual resistors). In this case you could use a jumper or a wire.