Voltage Is compared to pressure
Voltage is like pressure in a hydraulic or pneumatic system. If you have a small diameter pipe (or a high resistance wire), you will need more pressure (voltage) to get the same flow (current). Also, with more pressure (voltage) each unit of steam (charge) does more work.

By unit charge we mean some fixed charge, usually it is 1 coulomb, but for relative calculations you can take any fixed charge (say, 1 electron or 4200 electrons).