A feast where everyone brings along a plate of food.
The autocrat will generally supply bread and often cordial. Sometimes soup will be supplied too. Each person attending is expected to bring along a quantity of food suitable to serve 6-8 people (generally), ie bringing along enough food to feed you of one type, although you will share and eat some of many dishes. Remember that this is one dish per person, so if you invite a friend or bring along children, either ask them to bring food, or bring food for them.
Typical quantities of food to bring are:
Remember that Chicken and apples are pretty typical examples given for food to bring to potluck picnics, so sometimes we end up with 6 chickens and 3 bags of apples - very boring. Here is a list of other suitable /foods that don't require cooking. (also helpful may be a list of non-period foods.)
Of course if you have the time to cook for a potluck, we'll all love you. See recipes.
Remember to bring your own feasting gear, and that the food at the potluck will often be sitting around in your car/bag, then at the event, without heating or refrigeration facilities. (sometimes these are available, but ASK first). So to avoid food poisoning, it's important to use food hygiene standards. Dishes like pies, vegetables, fruit, sweets that can be served cold are good, while luke warm meat dishes can easily grow bacteria unless you can bring them straight from the stove/oven to the serving table at the event.
For info on what food to take along to a potluck feast (and what constitutes as period food), check out http://www.advancenet.net/~jscole/introfoodclass.pdf