A feast is one of the most popular SCA events. They can be the simplest of potlucks, or an elaborate eight remove feast. They can be themed (such as a night in Istanbul's first coffee shop), or just a generic, 'lots of food' feast. Most feasts are fully catered. They provide a great meal, generally presented in 2-3 courses (called "removes") with breaks in between for entertainments including dance, singing, music, games, drinking games, or cloved lemons, etc) and to allow food to digest. A feast will often follow (or sometimes accompany) Court. As it sounds, there is normally a lot of food at a feast, so it's normal to take a small portion of each dish, and go back for more of your favourites after everyone on the table has some. And don't forget to leave some room for the next remove - the first lot of food is normally only a warm up!
One subcategory of feasting is the potluck feast, where everyone brings a plate of food, and it is presented as a buffet. Some feasts may also be a fully catered buffet, rather than food brought to the table. This is more likely to be described as a supper. Read the feast flier carefully, and you should be able to tell the difference. If not, feel free to ask the autocrat when you book.
You are required to bring along feasting gear so you have something to eat your food off/with. It's also recommended that you pace yourself during large feasts or by the time you reach the fourth course, you won't be able to try any of the food in the last two courses!
Feasts also happen a lot easier if you help the autocrat out - remember everyone working there is a volunteer, and the main organisers may have been on their feet all day since early morning.