The refresh rate (or "vertical refresh rate", "vertical scan rate") is the maximum number of framess that can be displayed on a monitor in a second, expressed in Hertz.
The scan rate is controlled by the vertical sync signal generated by the video controller, ordering the monitor to position the electron gun at the upper left corner of the raster, ready to paint another frame. It is limited by the monitor's maximum horizontal scan rate and the resolution, since higher resolution means more scan lines. Increasing the refresh rate decreases flickeringing, reducing eye strain, but few people notice any change above 60-72 Hz.
