Last year, it was Caius, Newnham, Pembroke and Girton - I guess the same this year.
Basically, you have a few classes - maybe 3/4 a day - on some of the new topics that you'll be introduced to through the year which people often have difficulty picking up. You'll be given examples sheets to do, but it doesn't matter hugely whether you do them or not. Personally, I found it helpful, as I was moderately comfortable with things like suffix notation when they came up in the years lectures, but you're taught everything in term time lectures anyway so it doesn't matter if you don't take it too seriously.
Although, breaking news: Pembroke is NOT doing it this year.
Edit: Just thought I'd make this clear - this is only about the maths pre-course.
Its not compulsory to go, or so I gather from the email Emma sent me (engineering)
I had flights already booked so I'm not going up until 3rd October
you're correct. i gave admin a ring awhile back (when I was confused about it all). and they basically said its not compulsory, but they implied its kind of opt out, as opposed to opt in. so I think they expect you to go, but dont really mind too much if you don't.