Marketplaces are resource buildings in Tropico 3 and Tropico 4. Normally, when a Tropican citizen is hungry, they'll head to the nearest farm, ranch, or wharf to collect food, which can be very far away. If a marketplace is closer, the citizen will go there instead, saving on the trip. Marketplaces are stocked up with food from around the island by teamsters. Make sure you have enough teamsters employed; teamsters put a higher priority to transporting industrial products than food, so a marketplace may stop getting food shipments if all the teamsters are busy elsewhere.