The Flatlands is a region on the continent of Essos, located to the east of Westeros beyond the Narrow Sea. The Flatlands is a fertile area of open fields and plains, located around the Free City of Pentos and responsible for feeding it. Many magisters and nobles of the cities have estates on the Flatlands.
In the books
In the A Song of Ice and Fire novels, the Flatlands are bordered by the upper reaches of the River Rhoyne to the east, the Velvet Hills to the north and the Golden Fields to the south-east. The Dothraki have easy access to the Flatlands and find riding across it easy, so Pentosi nobles pay them heavy tribute so as to ensure they do not loot or sack any of the estates or villages in the area.