Plains are the second most suitable terrain to settle, providing a balanced yield of the two most important stats for cities: Food and Production. Like Grassland, they are also abundant in terrain features and resources.
Plains trades off population growth for increased production. A city built on plains will have good Production, especially if it has any Hills and Woods nearby, but growth will be slow until Farms are established.
The presence of Rainforests can help growth considerably, mostly due to the resources found there. The extra Food point for flatland tiles will make the difference between a normally growing and slowly growing city; still, you should look at every opportunity to build growth-enhancing buildings, such as a Granary.
A plain is simply a large, flat bit of terrain, with soil not as rich as fertile grassland but not as dry as desert. Thus, left alone (before all those tractors show up) plains tend to be overgrown with dry grasses and grains ... perfect for large herds of animals such as bison, antelope, horses, and barbarians.