Right before the meal, a small amount of each dish is put on a tray and offered up to the Buddha image.  In this photo you can see in the background someone walking the tray past all of the lay people.  Everyone touches the tray so they can be a part of the offering.  Although we know that the Buddha cannot receive our offering, we make it out of respect and generosity for the great gift he gave us: the teaching of the Dhamma.