2006 Hummer H3

It means there is more air than fuel as recognized by the oxygen sensor. Have you had any repairs, or maintenance performed, that would have included the air intake disconnected. Often times, the breather hose that connects to the cam cover, under the airbox, is disconnected at one end. You will need to loosen the clamp at the throttle body, and the two 10mm bolts that hold it down, to check this hose. It is not visible otherwise.