| Lambert's law is an expression of the diffuse reflection of electromagnetic radiation off a surface. It was discovered (obviously by Lambert) in 1760. It is most commonly used in relation to light. Basically stated, it says: the amount of light reflected off a diffuse surface is proportional to the cosine of the angle of incidence -- in other words, the angle between the normal to the surface and the vector to the light source. It is used frequently as part of the illumination equation in computer graphics.
Because it talks about the cosine, it is sometimes known as "Lambert's cosine law".
An intuitive understanding of why this is the case can be gained by considering a "square patch" of some material. If it faces a light source directly, then it captures the full intensity of the light source. But as you turn it on its side, the amount of light falling on the surface is less, since it doesn't directly face the light source, and a smaller amount of light is "spread" over the same area, so it's darker. Eventually, when the surface is parallel to the light source, the rays from the light source don't even "hit" the surface so it gets no light at all.
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