An animal (or a nest) is said to be cryptic if it is difficult to detect. It is the phenomenon of being camouflaged to resemble the surrounding environment. This can be achieved by a combination of colour, pattern, texture, shape and behaviour.
This Red Knot Calidris canutus incubating its eggs on its nest in the Siberian tundra illustrates the concept. The colours and patterns on its back match the surrounding tundra landscape remarkably well. Its behaviour, sitting completely stationary, means that there is no movement to give the position of the bird away.
The two photos were taken on the same nest visit, but in the top photo the incubating bird is being approached from behind.