For me the best and most basic example is a waterwheel used before more modern energy sources were discovered or implemented.
In a water wheel system water flows down stream then onto the water wheel where the weight of the water and gravity force the wheel to turn creating a powered drive system.
The hydro system is a lot like that except everything is sealed and we are adding a pump to replace the stream/river and gravity. The hydraulic pressure from the pump forces fluid into the vanes of the wheel motor which creates motion to power the drive wheels. Does that make a little more sense?