The frequency is the refresh rate of the display. It is directly related to the resolution of the screen; the higher the resolution, the faster the refresh rate.
For example, VGA has a frequency of 32 kHz (kilohertz), and a resolution of 640 x 480, while XGA has a frequency of 54 kHz and a resolution of 1024 x 768. The benefit of having a higher frequency is that the screen can display the image faster.
On a resolution that has 480 vertical pixels, it takes 32 microseconds for each line of the 480 pixels to display the image. On a display with 768 vertical pixels, it only takes 18 microseconds to display each line.
Therefore, higher resolutions, frequencies, and refresh rates are desired if you are using the display in a high motion application, such as video gaming, or precision engineering.