Helmet
Your mining helmet can provide partial protection from pesky monster damage.
It will block a small amount of damage from each hit.
Helmets are made of metal, and you will need to provide some ore as part of the cost to purchase the upgrade. The deeper you go in the mine, the better ore you will find and you can use it to forge better helmets.
Helmets are best used in conjunction with Armor, and the two upgrades complement each other very well. Fighting monsters is much easier once you have armor and a helmet.