You can store up to 400 items in the granary - 100 items in each of 4 slots.
Each slot has 4 menu items:
The red X ("Refuse") means the good won't be accepted into the granary
The green check mark ("Accept") makes it so the transporter gets the goods from the producer (ie bakery, gathering hut)
The green up arrow ("Stock Maximum") makes it so the transporter gets the goods from any other granary (with items not marked as Stock Maximum) instead of from the producer in order to try to fill the slot and keep it full.
The red down arrow ("Empty Stock") slowly empties all stock of that item, and does not accept anymore of that item.
A Granary only stocks food. The Warehouse is needed to stock Resources, Goods, and Luxury Goods. The Granary only accept the following objects: Berries , Cheese , Fish , Hop , Boar , Grapes , Wheat , Beer , Milk , Wine , Flour , Honey , Bread and Herbs