Hi, the drag cube, is the games representation of the crafts/parts dimensions, the game uses this info to calculate drag. Drag cubes for standard style and simple animated parts are calculated at every game start up, and written to the part database, once a DC has been written to the PartDatabase.cfg, this is the info that the game will always use.
Rather than repeating whats lurking elsewhere, have some links The first is the post that started my interest in tweaking DC's and the other two explain how to set it all up for parts that may, by default generate a procedural cube(BAD) or no cube at all.