A barn nail is an item sometimes found by foraging on your farm or as a prize on the Sales Bonus Meter.
Barn nails are used to upgrade the Barn, Silo, and the Back Porch Storage area.
They cannot be sold and they occupy space in your Barn. One barn nail has a value of 8 keys. 3 Barn Nails can obtained by trading 8 yellow ribbons at the prize tent.