A formal is a nicer dinner offered on Tuesday and Thursday nights in college. It's a three-course meal, and a great way to celebrate something, or just do something a bit special. You can buy wine for the meal in the bar. If you want to bring your own wine, you'll have to pay a 'corkage' fee in the bar for this.
The college expects formal standards of dress from those attending (jacket & tie, or equivalent). You should be seated in the hall by 7.25pm.