Basically it involves exploiting natural processes to "renew" the energy for us.

For example, you could have a large, elevated weight as an energy source. When it falls, it produces energy. Problem is, if you want to use it again, you have to get that weight up high. If you do it yourself, you are just going to expend more energy than you get out of the system in the first place.

But let's say a giant bird flies by and picks the weight up for you. In the end, more energy was still put into the system than you get out of it, but the bird paid for that cost, not you, so you don't care.

This is basically how renewable energy works, but for natural processes driven by the world or sun.

The Earth spins and the sun unevenly heats it, creating wind. We can use wind to spin turbines. The Earth and Sun are paying the cost to spin those turbines, but we are reaping the rewards.

Same for hydroelectric. Water falls and turns a turbine. The sun heats the water, evaporates it where it turns into clouds, the rains back down again, refilling the reservoir we use to turn the turbine.

So any time there is some natural process that operates on short time spans, and we can use that process to get energy out of it, that's renewable energy.