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Jitter is a small random offset added to a quantity. Raytracers usually make scenes more natural looking by adding a jitter value to the direction vector of the eye rays. This ensures that any sharp pattern captured by the rays is randomly broken up to give a dithered or antialiased appearance. This applies to shadow rays as well. Let's picture a scene containing an area light described as a 3x3 grid of point lights. If the point lights are not close together, we will get bands in the shadow of an object due to the fact that this shadow is actually nine separate widely spaced shadows mixed together. However, if the shadow rays are jittered, the bands will be scattered and a smoother shadow will be produced.

Jittered shadow
Left: a large 4x4 area light produces a banded shadow
Right: addition of jitter produces a smoother shadow

Siddhartha Chaudhuri, 2002