Basically a club (bat, heavy fighting stick etc). Ash, Oak or similar strong wood would used to fashion these simple weapons. Cheap and easy to make, they required little to no training - and were very popular among peasant Militias. In fact, during the English Civil War - a Regiment of Clubmen were recruited! The obvious usage was clear. Whack and beat your enemy down. Although not as romantic or elegant as other Medieval weapons, they were effective. A few example of metal Cudgels exist (and are often confused with Maces).