Renewable energy is exactly what it says -- it's renewable. Meaning, you can continue to reuse it or make more of it in a never ending cycles. Examples of a renewable energy could be trees (wood). You can keep growing trees and harvesting them.
Fossil fuels are things like oil or petroleum, they cannot be "grown" again, and are a one-time use.
Renewable energy has more benefits then that of fossil fuels because you can reuse renewable energy and keep the never-ending circle of energy going, unlike fossil fuels which can run out.