In the US, attending school through the twelfth grade, driving on the right side of the road, and obtaining a social security number are all mandatory. The phrase "mandatory requirement" is redundant.
A mandatory action is something that is required, obligatory, or compulsory. Like letting your Great Aunt Edna pinch your cheeks or passing gym to get your diploma. Mandatory is often used in opposition to optional. If you want to compete with the swim team at school, weeknight practices are mandatory, meaning you have to go, though the Saturday dawn swim practice remains optional, meaning it's up to you.