Bob in his post above is correct. It is a dioptre adjustment, but not for eyesight, it is to get the sharpest image for focusing and this changes with distance to the object. So if the image is blurry adjust the lever until it isn't, then you have a clearer view to focus with. You can still focus without making getting it dead sharp, the lever doesn't affect the actual focus, so if you are working quickly it can be something to intentionally forget about if you feel you are still seeing the focusing patch well enough, but certainly for nearer objects when focus is more critical because of reduced DOF getting used to using the lever quickly is worth doing.
Steve