Wikipedia
Locked breech is a firearms term used to describe the design of a breech reloading firearm's action. The term is important in understanding how a self reloading firearm works. In the simplest terms, it's one way to slow down the opening of the breech of a self-reloading firearm when fired. The source of power for the movement is recoil (see below)
The simple principle of the firing of a projectile is that when the propellant in the chamber (or shell casing) is ignited it burns quickly and for a very short time, creating pressure that pushed a projectile out of the chamber and down a barrel. The pressures involved in this are very high but for a very short time. If the high pressure gas was not confined for some time, it would do major damage, especially to people and even to the parts of a firearm that are not capable of withstanding those pressures. The feature of a barrel is that it confines the high pressure gas. The propellant burns for a very short time. Propulsion continues for a very short time while the gases expand and cool in the barrel. Because of the pressure drop breech block can be opened in a self reloading firearm due to the recoil inertia generation by the movement of the projectile.