Renewable energy is derived from sources that are easily renewed and replaced in a short amount of time or energy that doesn't deplete such as wind energy.
With wind energy as long as there's wind or even enough breeze the wind turbines will be able to turn and generate electricity and as long as there's sun then solar panels can produce electricity.
But with things such as natural gas or gasoline although those things are renewable it can take millions of years to make more natural gas and gasoline once it's been depleted.
So it's a good idea to have and use forms of renewable energy so we at least have a way to get and use the energy should those other sources of energy be used up.
It's also supposedly better for the environment to use renewable energy instead of non renewable energy but I'm not into that.