The phrase "to sit the exam" means to take or participate in an exam. It means that you are going to be tested on your knowledge or skills in a particular subject. Full definition
They claim pupils sitting the exams in one part of the year got different grades for the same marks obtained by those in another part of the year. (bbc.co.uk)
The results of the examinations will be used by the government to assess school performance, however, ministers have maintained pupils sitting the exams should not be made to feel like failures. (educationbusinessuk.net)
Much as we might love to, your teachers can not sit these exams for you or change the system back to one in which we were allowed to assess you ourselves. (tes.com)