Backscattered electrons (BSE) are primary beam electrons that, after interacting with the sample, follow a path back out of the sample.  They are generally considered to be emitted from the upper 1/3 of the beam analytical volume and their numbers (ie., relative brightness on the CRT screen) depend on the average atomic number of the material.  Thus, in multicomponent materials, like rocks, the individual minerals can be distinguished based on the fraction of electrons backscattered.