Hills go up and back down again. Plateaus go up and stay at a maximum level. For example, If you were measuring the elevation of certain points on a hill, it would look something like (in units): (1, 2, 3, 4, 5, 6, 6, 5, 4, 3, 2, 1). If you were measuring the elevation of certain points on a plateau, it would look something like (in units): (1, 2, 3, 4, 5, 6, 6, 6, 6, 6, 6, 6, 6).
