Most of everything we use today uses electricity. But how do we produce it? All power plants whether, coal burning, wind, nuclear, and hydroelectric, uses a force to turn a magnet in a coil of wire to power our modern lives. Coal burning, and nuclear both uses steam to turn the turbine while wind uses, well... the wind, and hydroelectric uses the pressure of flowing water.
Hydroelectric power is built into dams and build up water pressure until power is needed to be generated. At that point the water is released and the flow of water from the upper reservoir to a lower level using gravity to move the water past a rotating turbine which spins the magnet in the generator. The movement of the magnet through the coil induces a current in to wire which we call electricity. The greater the pressure built up behind the dam, the greater the force, and the more electricity that gets produced.