A ticket to get into the convention banquet. They are the one exception to the rule that at SF conventions everything is included in the membership. Banquet tickets are almost always sold separately.
The reason is that they tend to be quite expensive (at least 50% higher than a better meal in a restaurant) and many fans object to paying so much extra for a mediocre meal in order to have a prime seat for whatever speeches will follow. (It is normally the case that non-banquet attendees are allowed in once the meal is done to hear the speeches.)