Bloomers are a kind of knickers. They are very loose-fitting except at the waist and the ends of the legs. Traditional ones are roughly knee-length, although modern ones are usually much shorter.
They were popularised as a feminist dress-reform by the American social reformer Amelia Bloomer (1818-1894) in an article in her magazine The Lily in 1851. She proposed that bloomers fastened at the ankles could be worn under knee-length skirts, giving the wearer more freedom than the long skirts that were then the norm. They didn't catch on (since they were considered an unseemly imitation of men's trousers by many at the time). However, they later became fairly popular for women's athletic attire and swimwear in the early 20th century. They are not widely worn today.