Auto Fuel simply lets fuel that is placed in tool cupboard inventories be shared around to active fuel sources.
Simply put fuel in a tool cupboard's inventory where that item is within its radius and any powered fuel sources (lights, furnaces, etc.) will automatically take their required fuel item/resource (if it exists) from the tool cupboard's inventory when needed. Keep in mind what type of fuel each fuel source requires and make sure to keep the tool cupboard...