sit in for someone
phrasal verb with sit verb uk/sɪt/ us/sɪt/ sitting | sat
代替（某人）履行职责;代（某人）出席会议
to take the place of someone who would normally do a particular job or go to a particular meeting
Mr Baker is ill today, so Miss Dixon is sitting in for him (as your teacher). 贝克先生今天病了，因此由狄克逊小姐替他（上课）。