The mentoring system provides you with support throughout your time at Hatfield, advising you as to the best course of action should you have any problems.
Mentors are a mix of members of the MCR and SCR, as well as non-academics who have a great involvement in college. They understand the problems you may face with such a big change as moving to university, and will be able to point you in the right direction to get them sorted.
You will be appointed a mentor at the beginning of your first year at Hatfield, and will meet early on in Michaelmas term to get to know each other and address any concerns you may have. They will be available throughout the year for you to contact whenever you need to.
The system really is what you make of it, and you will not be forced to meet with your mentor regularly if you are doing just fine, but they are there as a support system should you ever need them.
It is important to note that mentors are there for pastoral and personal support, and will not not be able to offer academic advice or assistance. You will have an academic tutor through your department for help with course-related issues.