Calibre has two meanings that are commonly used. The historical meaning refers simply to the diameter of the interior of the barrel of the firearm. In other words, it's how wide a bullet the firearm would accept.
In modern parlance, with the advent of cartridge-firing guns, the term “calibre” has been broadened to mean the name of the cartridge that a firearm is designed to use. That could be in a standard calibre for a rifle or it could be an engage for a shotgun.