Spoiled food is the product of food that has been picked and in an inventory too long, without being put into a fridge. Food may spoil in cargo boxes, constructors, etc. but will not spoil in fridges. However, while spoiled food no longer becomes a viable source of food, it does become a useful crafting ingredient, especially in growth plots.
Each food item has a "Perish Time" which represents it's remaining time until becoming spoiled food.