A helmet was anything put over the head to reduce the trauma of a weapon strike.
The wealthy could afford a metallic piece closely fitted to the head. These went down to the base of the skull in back, covered the ears and cheeks while protecting the eyes. This would have been lined with leather. Most warriors likely had some sort of leather protection, possibly with bands of metal from the top of the skull down the sides to hold it in place but more likely with leather strips.
Simple helmets were common, made as they probably were from extra leather in a home. However, made of metal are rare, and likely only kings had them.