What you want is for the white areas of the original sheet of paper to NOT attact toner. Somehow the toner has to get onto the drum and then onto a sheet of paper. The drum selectively attacts toner. Then the sheet of paper gets charged with static electricity and it pulls the toner off the drum. The drum , or belt, is made out of photoconductive material.
Light is reflected from white areas of the paper and strikes the drum below.
Wherever a photon of light hits, electrons are emitted from the photoconductive atoms in the drum and neutralize the positive charges above. Negatively charged, dry, black pigment called toner is then spread over the surface of the drum, and the pigment particles adhere to the positive charges that remain. A positively charged sheet of paper then passes over the surface of the drum, attracting the beads of toner away from it. The paper is then heated and pressed to fuse the image formed by the toner to the paper's surface. Light reflected from blank areas on the page hits the drum and causes the charged particles coating the drum's surface to be neutralized.