Most resources and tools decay over time; decay is also accelerated when tools are used.
Resources stored inside a structure decay slowly. This is especially important for food, as it can mean the difference between having a good winter of massive starvation.
Some foods decay very quickly, like; Fruit, Berries or Raw meat, and Raw fish.
Some foods last longer like; Grain, Cured meat and Dry fish.
Your people will try to eat the food that will decay sooner to preserve more durable food for later.