What is a diopter setting in viewfinder?

I wear glass power of -2 but nikon d5000 has setting of -1.7 so is this a problem

with diopter can i use viewfinder without glass or what. Can i use an external 3rd party viewfinder in nikon camera what is the price

With those values, you can't use the viewfinder without glasses but you should try it first. Swing the diopter to one end (or the other) to see if it works. If it does, then good for you. If not, wear the glasses and adjust until viewfinder is clear. It is best to autofocus on a bright colored stationary object outdoors under the sun while adjusting the diopter.

You don't even need to look through the viewfinder to take a photo, though it helps.

I imagine the difference of 0.3 diopter isn't going to matter much, unless you're trying to manual focus. I have -5 vision, but I can still frame a shot close to where I want-autofocus does the rest.

If you absolutely need the correct diopter, check camera stores or eBay for diopter correction eyepieces that are compatible with your camera. (As with many other things, stick with actual Nikon equipment for best results.)