This is the number of times a second that a monitor updates what is shown. This is not the same as frame rate as the refresh rate can display a identical image many times. 60-120hz is common refresh rates.
Vertical retrace period
On a CRT monitor the period between each display of the screen is called the vertical retrace period. It's the time where the electron beam returns from lower edge of the screen back to the top to start to display again. If was the best period to change things such as palette colors and sprite data as that data it was not in use at that time. If data was changed outside the retrace period one could suffer from screen tearing.
With screen tearing the data from two or more frames show up at the same time on the screen. This can be avoided by making sure the update of the video data is done in sync with the refresh rate. There are a number of systems for that.
Latency / input lag
With V-sync the video driver waits until the monitor is done with it's current refresh cycle and start the vertical retrace period.
When the frame rate is high v-sync is used and when it is low it is off.