Albedo can be thought of as a basic "reflectivity colour" for a material.
For example, if a material has an RGB albedo with each component set to 0.0, it will be completely black and reflect no light; with each component set to 1.0, light would never lose energy reflecting off the material, which is physically unrealistic.
A comparison of various albedo values found in nature is available on Wikipedia.
An example diffuse material with a green albedo.
Materials which have an albedo attribute: