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 Frame = 1, Frame = 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 expression
me.time.frame to drive your nodes. For example use
me.time.frame - 1 for the index in the Movie File In TOP (set to Play Mode: Specify Index) to cache the for N images in a Movie File In TOP, into a Cache TOP.