On The Plains, a Player can use one mech to fight NPC monsters at the cost of some energy. When you win fights in the the Plains, the player will get commander experience, mech experience for the used mech, a bit of Max Energy, and Platinum.
You can move around The Plains by clicking the directional arrows in the upper left corner of the Plains interface. The more you move away from Sector 3, which is located in the center of the map, the stronger the enemies you face become, and so do the gains for a victory.
If your intention is to gain commander levels as fast as possible, it is not recommended to use the Plains; you should instead use your energy in the Battle Arena, as commander experience gains in The Plains are somewhat lower compared gains from victories in the Arena.