At cruise (constant load), it will continually cycle between a low value and a high, but overall should average out at 0.5V. This is the software in the ECU continually adjusting the fuel to achieve a stoichiometric A/F mixture. It cycles becuse it is reactive, and is so stupid that it can never actually learn what the correct value is.
Under load (boost), it will read high, becaue the ECU is programmed to enrich the fuel component of the Air / Fuel mixture.
When you back off (snap throttle shut, either in gear change, or sudden deceleration), it willl go to 0V (ultra lean) because the ECU has shut down the injectors for a short time.