This tag refers to tight form fitting athletic shorts composed of nylon and Spandex. The primary purpose of these shorts is to compress muscles (particularly the groin and thighs), prevent chafing, help remove sweat, keep muscles warm, and to keep male genitals in place. There are two similar appearing types of these, compression shorts and bike shorts.
On Danbooru, the most common version depicted are likely 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 used by some women to conceal their underwear beneath a skirt, especially in women's sports where their uniform has a skirt, such as field hockey and lacrosse.