Renewable energy is a type of energy that can be harvested from a natural source that is constantly available or a source that is replaced quickly by a process that occurs naturally. This includes solar power, wind energy, geothermal energy, and biomass energy, among others. It is also generally a clean type of energy in that it does not leave behind any footprint that may damage the environment.
Some forms of renewable energy are actually solar energy that has been stored. These include rainfall and wind, which are both short-term types of solar storage. Compare this to biomass which can take a few years to develop.
Fossil fuels are also naturally occurring, but they cannot be considered to be renewable as they have taken millions of years to develop and we are using them far faster than they can replace themselves.
A renewable source of energy can be used at its source to create heat or electricity, or it can be used in a process that will create energy further down the line. Windmills, for example, create energy at the source. Indirect use examples include a wind turbine or a solar panel.