Most artificially created "moving" images we watch (film, television etc.) are not actually moving but are a series of static images. The flicker fusion threshold is the speed at which a series if static images appear to become a steady moving image or when a flashing light flashes on and off quickly enough that we see a steady beam (flourescent lights).
The average flicker fusion threshold for human beings is 16 Hertz for 50% of test subjects, in other word if a light flashes on and off 16 times per second, 50% of people will see a steady light. This is why we have 50 and 60Hz refresh rates, where almost all people will perceive a steady state rather than flickering.
It's worth noting that we are more susceptable to this flicker effect in our peripheral vision than in our foveal vision, which is why we see the rainbow effect when we flick our eyes back an forth when watching a DLP projector.