General Question --- 35MM Camera?

I just got a 35mm camera for school and Im not 100% sure how to work it. On the "nose" of the camera, there are numbers 3.4 - 22 PK...i know they have to do something with the darkness and lightness of the photo. Do you use 3.4 when you take are taking a picture of somethint dark to make it more light? Or the opposite?