When a heat ray arrives at a body they may interact in three different ways:
# The body may absorb the heat
# The body may reflect the heat
# The heat may transmit through the body
Absorption and reflection are typically modeled as surface phenomena that occur within a fraction of a micrometer of the surface. For example, a highly polished piece of steel will be highly reflective, regardless of the material under the surface. Transmission on the other hand is a volumetric phenomena that is dependent on the properties of the entire thickness of the body. A glass window for instance must be translucent through its entire thickness for radiation to get through.
The term absorption is used to describe the change of heat into other forms of energy when it contacts particle or body. Only material particles can absorb heat rays, not elements of surfaces. For a given frequency of radiation, all mediums have a coefficient of absorption,  that represents how much heat will be absorbed per unit distance through a medium.