The chamber is that part of a firearm into which the cartridge is loaded. The chamber is also the place the cartridge is fired from. In a breech-loading weapon the chamber is directly behind and in line with the barrel. In a revolver there are multiple chambers contained in a revolving cylinder.
The typical number is 6, which is the reason for the nickname Six Shooter and indicates the cylinder has six chambers each holding one cartridge. Most rifles and pistols have a single chamber. To chamber a round means to load a cartridge into the chamber.