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. They've become a feature internationally in women's track and field uniforms.
Contrast with bloomers, which are a type of underwear constructed much like boxer shorts worn by women.