It is the renewable energy derived from the Sun.

Renewable energies are those obtained by the use of inexhaustible natural materials.
The traditional ones are  generated by exhaustible sources and as such, available in a definite quantity, no matter how large the reserve is.
You can take the fossil fuel as an example (like carbon or uranium).
The photovoltaic energy, like all renewables, has another essential characteristic: it does not produce any negative effect on the environment and it does not alter the climate with Earth global temperature variations.

The photovoltaic, with all other eco-compatible energy sources, represents the main resource of the future.