It's currently described as
Editors who meet all eligibility criteria are automatically enrolled as mentors
This reads to me like they become mentors without having to go to Special:EnrollAsMentor. What it actually means seems to be to give effect to the other two settings at https://en.wikipedia.org/wiki/Special:CommunityConfiguration/Mentorship.
It definitely needs to be redescribed, and ideally it would not be a separate checkbox at all but get set automatically if the days and edits fields are non-blank.