The horizontal refresh is the rate at which your graphics card is drawing lines when outputing to your monitor. It is closely related to your refresh rate and cannot be easily changed withing also changing the refresh rate.
E.g. for a 1024x768 display @ 60HZ refresh rate you will be outputing 768 lines 60 times per second, in actual fact their will be more lines as you also have to take into account the vertical sync and blanking regions which will probably add another 30 lines, this gives a horizontal rate of around 796 * 60 = 47.8KHz which is your horizontal refresh.
To carry this to the next step. You are drawing 1024 pixels per line, plus about 300 pixels blanking and horizontal sync. So your pixel rate is 1324 * 47.8KHz = 63.5MHz dot clock.
So the only way to modify your horizontal refresh would be to modify the dotclock and sync + blanking periods. Which will then cause issues for your monitor trying to sync with the signal.
CC