Hello!
I am going to write this as easily understanding as I can. If you have any further questions regarding it please let me know.
My answer : A plain is a landmass that is flat or gently rolling and covers many miles. There are also different types of plains such as grasslands and steppes.
How plains are formed is that they are sediments which transport all kinds of materials such as rocks,trees,rests and many more. This is for the plains on the coast which rise above sea level but some articles also refer to the molten of the lava which created solid,flat terrain but also erosion can do it. Mountain runoff also causes the erosion for the terrain..
