请解释“曲水流觞”

曲水流觞
qūshuǐ-liúshāng
英语意思：[(of a gathered crowd) drink water from a winding canal with one wine cup floating on it so as to wash away ominousness]
中文意思：古民俗,每年农历三月在弯曲的水流旁设酒杯,流到谁面前,谁就取下来喝,可以祓除不吉利