Pre-Filling is a feature used in a few OPs, like the Cache TOP, Cache SOP, and Texture 3D TOP. It is used to entirely fill up the contents of an OP that usually only fills a portion of it's contents each cook. For example a Cache TOP will normally only cache 1 image per frame. By pre-filling this OP you can cache all of it's images upon load up.
The pre-fill is done by cooking the node's input(s) at $F =1, $F = 2 etc, up until the number of cooks required to fill up the OP are reached. To generate different information for each cache item, use the variable $F to drive your nodes. For example use $F - 1 for the index in the Movie In TOP (set to Play Mode: Specify Index) to cache the for N images in a Movie In TOP, into a Cache TOP.