The Engine Computer alternates from commanding a too rich fuel / air mixture to a too lean mixture a couple of times per second, then it expects to see corresponding changes in what the oxygen sensors detect for unburned oxygen in the exhaust system. In this case there is too much unburned oxygen in the driver's side exhaust and the computer is unable to correct it. Could be a vacuum leak that is affecting one side of the engine more than the other, a partially plugged injector, low fuel pressure, or a misfiring spark plug. Those are some of the common causes. The fault code just gets you to the circuit or system that needs further diagnosis.