A parfocalled microscope objective lens means that on a biological microscope when you move up in magnification from one objective lens to the next the specimen remains in focus.
If you have a microscope objective lens that is not in focus when the other lenses are, you can adjust the lens by following these steps:
Remove the objective lens from the microscope.
Unscrew and remove the outer covering from the lens.
With the cover still removed, screw the objective lens back onto the microscope. Move down to the lower objective lens power and ensure that the objective is in focus when looking through the eyepieces. Move back to the lens that needs to be parfocalled. Rather than turning the focus knob to focus the lens, adjust the small screw on the side of the lens until the image is crisp and clear.
Remove the microscope objective from the microscope, replace the metal cover on the lens and screw the lens back into place on the microscope. Learn more about how to parfocal microscope objective lenses here.