Hydro power is commonly understood to mean 'power generated by moving water'.

That's a pretty broad description, so we'll try to give some more specific examples.

If water falls it converts 'potential energy' into 'kinetic energy', and this 'kinetic energy' can be harvested using a whaterwheel or a turbine.

If the waterwheel or turbine is connected to a generator or alternator you may convert this motive power into electricity.

This happens on a giant scale in places like the Niagara Falls, and on a much more modest scale in backyard hydro turbine (also called micro-hydro) installations, and at every scale in between.

Hydro based power generation is very nice because it usually is a very predictable and constant source of energy (as contrasted with wind and solar power which are extremely variable), though flow changes can cause variation these usually are only seasonal variations.

Because of these nice properties the countries that have access to abundant water and height differences are relying more and more on hydropower for their needs.