What is the drum unit? The drum unit transfers the toner to the paper. The laser printer puts the toner on the roller called "The photosensitive drum" inside the drum unit, and then prints letters and images by fixing toner to the paper by heat and pressure.
In other words; the toner powder is negatively charged and is magnetically transferred onto the positively charged drum roller, which then creates the print on the paper that passes through the printer.
