It was the job of a sheriff to deal with legal matters on a local level. He also collected taxes and turned them over to the Exchequer (after taking his share). Initially, sheriffs performed the duties that would later be the province of itinerant judges, coroners, and justices of the peace.
In some cases, as the royally-appointed overseer of the county, it fell to the sheriff to make sure that the king's hunting preserves were protected; and when the monarch was in his shire, it was the sheriff's job to see to it that his king had all the necessary provisions (food, comfortable lodging, etc.).