A renewable energy source is simply one that won't run out. This means that you can consistently use it to produce energy and unlike fossil fuels, it won't use up supplies. For this reason, renewable energy often comes from cycles (like that of the tides) or from a constant in the environment. An example of a non-renewable energy source is fossil fuels. We will one day (in the not so far future) run out of fossil fuels as they take millions of years to produce, and we use so much of them up.
Renewable energy sources are getting more and more in demand as we run out of fossil fuels and realise their effect on the atmosphere. Here are a few well known examples.
Solar energy - this is where the energy from the sun is harnessed to produce electricity, or directly heat houses. By creating a panel with a large surface area and heat absorbent surface, it can absorb a large amount of the suns thermal and light energy to either heat up water in pipes (which will then heat houses), or to create electricity by "exciting" electrons (small particles which play a large part in transporting energy) in silicon using the sun's rays (or photons). However, solar energy can be a little unreliable as it requires clear skies and sunlight.
Wind - you may have seen large wind turbines along coastlines. This is because coasts receive a lot of wind, which can provide a force strong enough to turn a large turbine - then connected to a generator allowing us to harness the energy of the wind and turn it into electricity. Again, due to it relying on the weather, this is not a method that will 100% produce energy all the time.
Tidal - this is perhaps one of the most reliable methods, as we can rely on the tide coming in and out each day. It works by building a large dam across an water body affected by the tide. In order for the water to move in the direction it wants to, it has to push it's way through a turbine. The turbine is moved by the water and in turn drives a generator.
This is similar to hydroelectric, where instead of the tide, the energy of water flowing downhill is used to turn the turbine.
I hope this helped, let me know if you need any more information or something explained a bit better:)