The Freshers Fayre and Icebreaker sessions are designed to ease you into student life. You are not obliged to attend either of them, though there may be course related taster sessions on your timetable that are mandatory but it depends on your university/SU.
The Freshers Fayre, as advised above is a mixture of stalls and entertainment where local businesses give you deals and discounts to use as a student. There's usually a lot of freebies to grab too and your university clubs and society representatives will also be there where you can find out more about what they offer and how you can sign up. If you're interested in joining one, it is worth going as you'll get to meet the current members and get a real insight into what to expect.
The icebreaker sessions, again are designed to help you meet new people and make some connections.