A reference designator is used in PCB design to help identify components across documents and PCBs. The designator code is made up of a letter that stands for the component name — such as R for resistor — and a number that corresponds to the specific component in the schematic — R1 and R2 if your project has multiple resistors.
Do you need to use reference designators?
There's no need to use reference designators, but they can help keep you organized when working from a schematic. You'll also encounter them in consumer electronics devices, so it's helpful to be familiar with what all those letters and numbers mean.