Can anyone clarify what is meant by the message  "Sensor Bias Too Big" and how to rectify.

It is a message that precedes a prompt to recalibrate IMU if I am not mistaken. Try recalibrating.

PS: In my case the motors will not start until I recalibrate.  I once got the message directly after a compass calibration.  It did not matter that I'd recalibrated IMU the day before.

I've had this issue as well sometimes. Tried turning it off and on, re-calibration (multiple times), moving it around etc. Nothing seems to fix it just after a few reboots it starts to spontaneously work can be really frustrating when you're trying to get a shot in a hurry.

My bird was experiencing this also. I did and IMU calibration indoors and I was able to fly afterwards.
I noticed this happened after the latest (feb) update.

Do the take off floor need to be 100% leveled? that is really realistic in many cases.

I would like to know what causes this error or what is really means.

Usually caused by magnetism,,, it is a discrepancy on the range of IMU calibration or compass being outside acceptable range... It might go away if you initiate bird away from what might have caused the issue...without even having to recalibrate.l.  Sometimes the IMU might need to be calibrated, or the compass or both...

I would make sure, and not take a chance by changing location of where you do the compass calibration and IMU if it suspect of causing the error, and re do both!