Superdetailing is a general term intended to apply to all refinements of a model railway, and includes such aspects as equipment (locomotive) detailing, equipment and rollingstock weathering, lighting, non-rail moving items [such as moving road vehicles, travelling circus rides or construction equipment, operating mills, and so forth], but this article will confine itself to general layout scenery superdetailing basics or essentials.
"Basic scenery" would normally consist of ballasting, hills, rivers, tunnels, bridges, trestles, general landscape, rock faces, trees and bushes, buildings - everything that eventually covers the bare table top, and also backdrops.
"Superdetailing" is designed to provide that additional detail to the "basic scenery" that makes onlookers want to examine the layout more closely regardless of whether any trains happen to be running or not. In fact, with enough to look at and to think about, moving trains almost become a secondary reason for stoppng to look at the module.
My article The Credible Model discusses the basic principles involved in creating a believable layout.
This article goes into superdetailing in more, well, detail.
At one time, these enhancements pretty much had to be scratch-built, but today there are many products, such as Woodland Scenics, Osborn Kits, Preiser, and so forth, available in all scales, even though scratchbuilding is still a fun (and an economy) option for all but the most finicky items that can now be produced as laser kits at a reasonable cost.
In deciding on what and where to superdetail, obviously those areas of the layout most likely to be scrutinized by visitors should receive priority attention.
People, Station Areas, Action Scenes
Vehicles, Planes, Vessels and "Street Furniture"
Buildings and Structures, adjunct Scenery