A nail is the second ingredient for your base fortification and security.
Nails are also used to modify certain weapons, and are good multi-purpose tools. You can use nails to make barricades in your base and to create wall pieces (door piece, door frame, etc), to further extend and upgrade your base.
Nails are pretty common in zombie corpses, and someone who killed a ton of zombies will be likely to find a ton of nails, thus searching for nails is more like deliberately hunting for zombies. However, you can also find nails in cabinents, crates, and drawerers of certain houses/structures.