Nuclear power is a form of energy that is created by splitting atoms of uranium in special machines called nuclear reactors. This process is called nuclear fission and it releases a lot of energy in the form of heat. This heat is used to turn a turbine which generates electricity, just like a regular power plant.
Nuclear power can be considered renewable because if the reactor is set up and run correctly, the uranium atoms do not need to be replaced. That means, once the reactor is built, the energy it produces will not run out. This is different from burning fossil fuels like coal and oil which are not renewable because when these fuels are used up, they cannot be replaced. That is why nuclear power is sometimes considered a form of renewable energy.