Toner is like a cartridge of ink mostly used in laser printers. It has the ink in powdered form. If your toner is finished, you can buy a new toner from market and replace your current one. The printer will start printing again.
The toner contains the ink which the printer needs in order to print pages. If the toner is empty the printer cannot print anymore. When buying a new toner take a look at its size to make sure the printer works with the new toner.