The toner cartridge contains powder (toner, instead of liquid ink used in ink cartridges) used for creating letters and images.
If your printer requires you to replace your cartridge, this would be what you need to replace. It's usually as long as a forearm, and for some printers such as Brother, you may need to install it inside a drum before installing it inside your printer. For the most part, you can use the same drum you'd been using in your printer already, until your printer requires you to change the drum as well. You need to replace the toner cartridge when the toner is running short or print quality becomes poor because of toner deterioration.