Radiant heat is thermal radiation between two surfaces. Thermal radiation occurs when a material transmits infrared radiation, thereby warming a second material, without causing the air around the materials to be heated.
The sun is a good example of a powerful source of radiant heat, as it heats the earth exclusively with thermal radiation.
In buildings, wood stoves and radiators transmit a lot of radiant heat. Heat sources like these also emit convective heat, meaning that the air around them is heated as well.
Heating with pure radiant heat can cause indoor climate problems, as the large differences in temperature can give discomfort. Pure radiant heat is typically used as heating in athletic halls, workshops, storehouses, etc, so only the areas that are used are heated.