The drum unit is what holds the toner cartridge in some printer models such as Brother, it's about as long as a forearm. It 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.
Please be aware that drum unit is also consumables and it has limited page yield, you will need to replace it when your printouts appear excessively blackened, or you get a warning message asking you to replace drum. Usually a drum unit can last 3-4 times longer than the toner.