Simplify the following text for a Grade 7 native English-speaking person.
Original: In computer security , mandatory access control ( MAC ) refers to a type of access control by which the operating system constrains the ability of a subject or initiator to access or generally perform some sort of operation on an object or target .
Simplified: In computer security , a mandatory access control ( MAC ) means a type of access control by which the operating system changes the way a subject ( e.g. a user or a program ) can access or do some sort of action on an object ( e.g. a file or folder ) .