Buruma is an English to Japanese to English recursive transliteration of bloomer, usually identifying an over-panty or public garment, whose purpose is to cover the panty and crotch area while leaving the maximum maneuvering room.
Typically this is used as part of a gym uniform, paired up with a t-shirt. Though in real life they are rarely part of a gym uniform these days since they're considered old-fashioned, they still show up in anime since they are fetishized by otaku.
The buruma has become a feature internationally in women's track and field and other sports uniforms. Though the design and materials may be different, the "racing buruma" as they are known in Japan, serve the same purpose and have the same brief shape.
Contrast with bloomers, which are a type of underwear constructed much like boxer shorts worn by women.