Means 'The Green Province'. It was the name of the rolling plains of Gondor between the Anduin and Isen before they were given to the Éothéod by Cirion the Steward of Gondor as a reward for their help in the Battle of the Field of Celebrant.

After the Éothéod made their homes there, the land was renamed Rohan and the people called the Rohirrim by folk in Gondor. The Éothéod referred to their own new lands as the Riddermark or the Mark of the Riders and their folk were called the Eorlingas.

To read more about Rohan and the folk who lived there, please see the link above to the Middle Earth article called 'A History of The Rohirrim'.