60Hz is the 'flicker fusion rate', meaning if you were changing the frequency of a flashing, stationary LED, ~60Hz would be the frequency where perception transitions from visibly flickering to apparently continuous. It's a lower threshold for refresh rate in the human eye. When you have complex refreshing images (a 2d computer screen rather than a point-like LED, diverse motion, depth, etc), you are likely to notice flickering (or tearing, jittering, non-smooth motion) if the refresh rate is near this minimum.