the refresh rate is how many times per second (cycles per second are measured in Hertz or Hz) the screen is redrawn. the faster it's redrawn the less your eyes notice the flicker, and the less eyestrain you have.
right now I am running a system with Win98 installed, and due to technical difficulties (meaning I have no idea yet how to fix it) I'm stuck at 60Hz, after getting used to 85Hz on my other puter. my eyes notice flicker and I'm rubbing them more often.
look at the back of your monitor for display settings it supports, or choose the "hide refresh rates my monitor cannot display" in your "display properties" (if using a Windoze system). that way you won't damage your monitor if you set the refresh rates too high.
~Branson