Dark spaces cast by the presence of a solid object blocking the flow of light. Often helpful for telling where your character is going to land.
Shadows are an interesting tool, in that they are an absence that indicates a presence. They serve as a very subtle but powerful tool to a designer. In games, shadows can be used to help assert the position of entities, warn players of incoming danger, or give important story clues.
Questions with this tag deal with both real simulation of shadows via light projection (as in most 3D games), as well as faux simulation done without actual light systems (such as might be done for a 2D platformer).