Refers to a flat, wide area of land, often in contrast to mountains or hills. Common in geographical descriptions.
The Kanto Plain is the largest plain in Japan.
このあたりは山がなく、平野が広がっている。
There are no mountains around here; a plain stretches out.
The Kanto Plain stretches out over a wide area.
There is a wide plain in this region.
We crossed the vast plain by train.