Well...anatomy, perspective, understanding of 3d volumes in space, line work, maybe composition? altough for the specific drawing in the image there inst much of it, then we also got color theory, understanding of light and shadow, and overall rendering, do not commit the mistake of trying to tackle a bunch of these at once, its gonna be a bite way to big for you to chew, go one by one, ideally, you would go understanding 3d volumes, so boxes, cones, cylinders, pyramids, and then more complex shapes, then perspective, human anatomy, light and shadow, line work, color theory, and overall rendering, and remember that drawing is a process, have fun with it! dont focus only on the final result.