desired idle is the target idle that the ecm would like to see. idle is the exact reading that the crankshaft position sensor is seeing. several factors can make the readings different. slight vacuum leaks, carbon in throttle body holding the valve open, any slack in timing chain or belt, etc... usually not that much difference in desired idle and actual. if the threshold is exceeded then you will get codes for excessive idle speed or idle speed less than desired, and those codes will help you figure out which direction to go.