It refers to the energy transmitted from the sun to the earth through the atmosphere.
It passes in the atmosphere in form of a beam of short wave rays (solar shortwave radiation)
Solar radiation is received in a place during the day in a form of light and it is converted into heat at the earth’s surface
The amount of solar radiation received in a place also varies from time to time. The air, clouds and the ozone in the atmosphere absorb some of this energy while dust particles and clouds reflect and scatter the energy into the space.
The earth, therefore, absorbs only a fraction of the energy emitted by the sun. The nature of the earth’s surface (Albedo) is used to express the ability of a surface to reflect insolation