Transmission is the process by which, "radiation is propagated through a medium. Measured as transmittance, it is the ratio of the transmitted radiation to the total radiation incident upon the medium." Electromagnetic radiation flows through the atmosphere everyday. Most of it is absorbed as seen on the absorption page. Some of the radiation makes it though. This is what is called transmission. The atmospheric windows, as seen in the bottom graphic, are caused when atmospheric gases and particles do not absorb the radiation. Visible light is one of the wavelengths that makes it through generally unabsorbed.

 Below is a graph showing the different gases and the amount of energy absorbed and transmitted.

Atmospheric radiation, an online guide