This operator returns the amount that the current bounce level will contribute to the final pixel color.
A primary ray is one sent directly from the camera. It has a bounce level of 0 and a bounce-weight of 1. After a single reflection, the ray bounce level is 1. After two reflections, the level is 2, etc. The weight of each subsequent ray level is determined by the type of surface it encounters.
For example, assume the primary ray (of weight 1) hits a 0.1 reflective surface. Reflections in this surface will contribute only 0.1 to the final pixel color. If a second 0.1 reflective surface is hit, the ray weight will drop to 0.01.
This operator can be used to do less work for rays which will not contribute very much to the final output color (i.e. compute fewer octaves of noise, etc.).