There are Pallets for each of the four non-premium currencies (Food, Salvage, Lumber, and Fuel) and the purpose of all of them is to generate resouces. They are particularly effective at supplementing the resources that you collect from Scavaging. The production rate of the Pallets will increase with each level and when a Pallet has reached its maximum capacity you must collect the resources from it otherwise it will not continue to produce them.