The Nikon DK-17C +1.0 Correction Eyepiece is also known as a diopter - it functions in the same way a contact lens is used on your eye. Correction lenses provide near or far-sighted users a clear viewfinder image without the use of eyeglasses. It allows you to optimize your SLR viewfinder's sharpness in conjunction with your individual eyesight.
It is highly recommended to consult an eye professional to determine what strength diopter is correct for you. However, you can try different reading eyeglasses at your local pharmacy to get a "ballpark idea" of which one is right for you.