Radiation coming from the sun generally has short wavelengths and high energy. When this radiation reaches the Earth, some of the energy is absorbed by the air, land and water. As a result, the radiation that is emitted from the surface of the earth has longer waves and less energy.
You will learn later that gases in the atmosphere absorb these longer waves and create the greenhouse effect. However, some wavelengths are not absorbed by any gases and pass through the atmosphere and back into space.