According to BodyBuilding.com, a preacher curl bench is a bench designed for the purpose of performing preacher-style bicep curls. A preacher curl also requires the use of either a dumbbell or an E-Z curl bar.
ExRx.net states that a preacher curl is performed by sitting on the preacher curl bench and grasping the curl bar with a shoulder-width underhand grip. The bar is then raised until the forearms are vertical. Then the bar is lowered until the arms are fully extended. The preacher curl primarily targets the biceps and forearms, but the wrist muscles are also used to a minor extent as stabilizers.