Preflashing is giving the paper negative a slight, even exposure (in the darkrooom, prior to loading the film holders) whose purpose is to help improve the paper's shadow exposure without adding much additional highlight exposure. This is used to control what would otherwise be excessive contrast in harsh, direct sun.
Originally Posted by DimDim
I give enough preflash exposure so that if you then develop the paper without the in-camera exposure it would be a very slight gray in tone.
Some people preflash in the camera by placing a frosted piece of white plastic over the lens and make an exposure several stops below the pictorial exposure.
Other people use yellow filters over their lens, on the theory that it filters lots of the UV/blue that contributes to harsh contrast on photo paper.