Renewable might not be the right word really. Think of it in terms of when an energy source will run out. Take a disposable battery. You get an hour or two of running something small, and then it's empty. Plus it took work on our part to put the energy into the battery. Now think about the sun. Technically it will run out of energy in billions of years, but for our life spans there's an infinite supply of energy, and we don't really have to do anything to cause the sun to provide energy (aside from devising ways to collect it). So renewable energy would be a source of energy that is going to keep producing without us doing anything to make that source produce. We don't have to do anything to make the wind blow, or waves move, or the insides of the Earth hot. So if we collect energy from those sources it will renew itself.