The First In, First Out (FIFO) system is used for old and new foods that need to be rotated in the fridge.  Specifically when food and box cartons are labeled with an “opened” date or simply the expiration date.  Now in a home fridge (versus restaurant) you probably aren’t going to date your food since it’s your fridge, but you can watch for expiration dates.  FIFO for a home fridge is more to be aware of food that is opened, old, or needs to be used up.  FIFO = food that goes in first, needs to be out first.