Grassland is one of the most settler-friendly terrains in the game. It is found in large sections of the world, particularly close to the middle of the map. Additional terrain features, such as Woods and Marsh are abundant on it, as well as many Luxury Resources.
Grassland ensures strong population growth for cities - especially with farmland developed - but does not provide Production by itself without Hills, Woods, or certain Resources. A city with with an abundance of Grassland Hills will still have better growth than flat Plains, but more restricted options for farming.
Grasslands, blessed with rich and fertile soils, have been the heart of many civilizations, providing the food and open frontiers so necessary for expansion. Unfortunately, every barbarian also realizes this, and so grasslands have been fertilized with blood over the centuries.