Metal is an important resource later in the game. Many early buildings and items will not require much metal, but you will still want to try to collect a lot of it because it will be important for later buildings and items.
Metal can be harvested from the Mine, and is stored in your town's stockpile when collected from those buildings. It can also be earned when you attack or defeat certain enemies.
Metal accumulates relatively slowly and is valuable, so use it wisely! You can hold up to 4000 metal at first, but can increase that capacity by building Metal Storage units.