What people have said so far is correct: there is a white plastic dome that slides over the metering cell for incident metering. Depending on which model it is there may also be two settings, one for low light/indoors and one for oudoors/higher light. Simply set the film speed, point the metering cell at the subject and press in the lever or button on the side of the meter then make a note of the number on the scale the needle settles on. For incident readings, point the meter (with the white dome in place) at the camera and do the same. Having made a note of the number, set that number on the scale on the outside of the revolving wheel. You can now read off all the combinations of shutter speed and aperture that will give correct exposure.

Again depending on which model it is it may require a mercury battery - which you can no longer buy. If it's a more recent model then it takes SR44S. Always use SR44s, not LR44s which are the same size but will give inaccurate readings.