First in, first out means the units that arrived first are used or sold first. As a rotation rule it keeps old stock from sitting at the back of the shelf. As a costing method it values the cost of goods sold at the oldest purchase prices.
For food with expiry dates, FIFO is a reasonable default but not always enough: received order and expiry order are not always the same. Most perishable distributors rotate by expiry (FEFO) and value inventory by another method, such as weighted average cost.
APFoodz rotates perishable stock by expiry and values inventory at weighted average cost.