The front O2 sensor reads the burned or unburned fuel and tries to regulate what is going into cat. The rear O2 sensor checks that the exhaust is clean. Below Threshold means the exhaust is not as clean as it should be. Where the problem is has not been given to you by any other codes. Had you left the code in the system it will usually clear by itself if the O2 sensors see what they like but if not another code such as "running rich" might apear