This tag refers to tight form fitting shorts composed of nylon and Spandex. These are athletic shorts which reach about mid-thigh. The primary purpose is to compress muscles (particularly the groin and thighs), prevent chafing, help remove sweat, keep muscles warm, and to keep male genital in place. There are two similar appearing types of these, compression shorts and bike shorts. Both types may be used without underwear, depending on if they were designed to act as underwear.
On Danbooru, the most common version depicted are compression shorts, which are used for a variety of sports and athletic activities. Compression shorts are similar to bike shorts, but aren't designed with biking in mind and thus lack things like padding to reduce discomfort from long rides. Compression shorts are sometimes used by some women to conceal their underwear beneath their skirts, especially for those in women's sports where their uniform has a skirt, such as field hockey or lacrosse.