Does the scene as viewed through your viewfinder appear slightly blurry, even after you've had the camera auto focus? Do you cross your fingers and hope the photo is in focus, trusting the camera rather than your own eyes? Your diopter may need adjustment.
The diopter adjusts what you see through the viewfinder -- it does not affect the focal function of the camera or the resulting images. The diopter will provide slight changes in focus for the photographer.
To the right of the viewfinder is a small dial. While looking through the viewfinder, after composing a scene and pressing the shutter halfway down to focus, slowly adjust this dial. If nothing appears to happen, turn it the other direction. As you do, the scene should become more or less clear, depending on the direction you move the dial. Continue to adjust until what you see is clear and in focus. If you wear glasses, you may be able to shoot without them, depending on the strength of your prescription. Try adjusting the diopter with and without your glasses on to see what works best for you!