:to take the place of someone who is not present代替
The President could not attend the meeting, so the Vice President is sitting in. = The Vice President is sitting in for the President.由于总统无法出席会议,副总统将代为出席。
2
sit in on (something):to attend (something, such as a class or meeting) without officially participating旁听(课程);列席(会议)
She's been asked to sit in on the meetings.她被邀请列席会议。
I would like to sit in on one of your classes.我想旁听你一节课。