There are 2 basic processes: tectonics which builds the earth and surface processes which tear it back down again. Tectonics include volcanoes and earthquakes, and tectonic processes are responsible for plate motions, mountains and so forth. Surface processes include weathering, erosion, and the transportation of sediments, and are responsible for rounded hills, valleys, and places like Kansas.
A good local example in your area (Arizona) would be the Grand Canyon. Tectonic forces are causing the uplift of the sedimentary rocks exposed in its walls, and the erosional power of the Colorado River is causing the downcutting which is creating the canyon and exposing the rocks in the canyon walls. In keeping with the Third Law of GeoFantasy, all of these weathered materials end up at the beach, where they are deposited. Through the process of lithification, they may then be converted into new sedimentary rocks, which can then be uplifted and eroded again.
The equilibrium part relates to how these 2 processes are more or less in balance. Sure, in any given place either tectonics or erosion might be winning, but on a global scale (as well as a geologic time scale) they are in balance, and will continue to be until one of two things happen: either the hydrosphere will go away and we'll have no water to perform the magic of erosion, or the earth's internal heat will die, effectively ending the earth's ability to uplift mountains above sea level.