In most laser printers, the cartridges contain a large empty space containing a very fine dry powder called toner. The toner is passed on to other components, which eventually melt the toner onto the paper to be printed. Printers have different ways of detecting when the toner runs out. Some Brother printers let light pass through the cartridge, and a large amount of light passing through indicates that there is little toner left. They can be fooled by covering the 'box' on the side of the cartridge with black tape. However, there are cases where the cartridge uses a different, more sophisticated method and simply stops printing when it runs out of toner.
Toner is sold in bottles that you can use to refill your cartridge. Make sure you get the right toner for your printer. One way to change the toner in some Brother cartridges is to take out the rear gears, remove the stopper and then put everything back together and set the gears so that the printer thinks it is a new cartridge. Sometimes you even need to buy new parts when you refill the cartridge this way. But there is an even simpler and quicker way to replace the toner.
In any case, so that you don't have to worry about the wrong cartridge or toner, you can always choose a specialist who does just that. This way you will avoid wasting time thinking about whether you are doing everything right and reduce the risk of damaging the cartridge you have.