RBI means “residual bulk image”, where ‘bulk’ means the substrate of the sensor chip. when you run a CCD super-cold, charge from the photosites can leak into the substrate and stay there, such that after you read out the chip and reset the photosites, the charge in the substrate can then leak back into the photo sites, mimicking the arrival of new photons. thus you get a ghost image of the previous exposure in your current exposure.
the RBI pre flash (also called “flood”) floods the sensor with IR light and then reads out (and discards) the flood image before the next real image is taken. charge from the flood is still trapped in the substrate, but hopefully after the flood the charge is uniformly distributed rather than being an image of something.
if you turn on RBI preflash for your lights you also need to do it for your darks and biases, because you need to get rid of the flood RBI somehow. an interesting thing is that you can often see semicircular swirls in the RBI darks - these are caused by slight differences in the crystal structure of the silicon as it is grown in cylindrical ingots. the wafers are cut perpendicular to the long axis of the ingot, and then the chips are cut from the circular wafers into squares.