To resit an exam is to take it again, usually because you failed or your mark was not high enough. British and some other school systems use it; American English prefers retake. As a noun: I’ve got a resit in June. Though actually, sit here is the British verb for taking an exam (“sit an exam”), so resit is literally “sit again.” It is ordinary in UK university emails, a bit foreign in US ones. It is not reset (to put a machine back to the start), and it is not a casual word for studying the chapter one more time — you resit the test itself. In Taiwan classrooms you will hear 重考 more often than this English word.

重考（英式；美式多說 retake。台灣「補考」較接近補一次，不一定等於整科重考）