Mountains are a type of terrain. Only Mechs, Infantry, and air units can travel over mountains, and Infantry still suffer a movement penalty while crossing them, along with Mechs in the pre-AW games. In Fog of War, Mech and Infantry units increase their vision range by 3 while they're on a Mountain. Mountains also offer good defensive cover.
In Game Boy Wars 1/2/Turbo, the APC could scale mountains, but requires all 6 Movement Power for each tile.
In Game Boy Wars 3, all Unarmored vehicles except the Constructor and Supply Truck could move on Mountains, not requiring all of their Movement Power for each tile, but still a high amount. (4 to be precise.) However, Mountains also hinder Helicopters with a 1.5 Movement Cost.