At Christ's, Formal Hall is available six nights a week and you can sign up to attend on evenings when you'd like a special served meal with college friends or visitors rather than going to the normal College canteen (Upper Hall). Think of it as an in-College equivalent of going out to a restaurant, except that at just under £10 for three courses plus coffee, it's cheaper than equivalent Cambridge restaurant prices. See the meals page for more information about the various options for food, since as well as Formal Hall, there's the self-service canteen, the Buttery cafe-bar and "gyp rooms" in the accommodation too.
These pictures show a fairly standard Formal Hall meal, but sometimes special events are organised by the students too - there may be a theme that people dress for etc. You don't have to go to Formal Hall meals if you don't want to - it's just one of the choices for eating in College.