A breechloader is a firearm in which the user loads the ammunition (cartridge or shell) via the rear (breech) end of its barrel, as opposed to a muzzleloader, which loads ammunition via the front (muzzle) end.
Modern firearms are generally breech-loading – except for replicas of vintage weapons. Early firearms up until the mid-19th century, however, were almost entirely muzzle-loading. Mortars are the only muzzleloaders remaining in frequent modern usage.
Breech-loading provides the advantage of reduced reloading time, because it is a lot quicker to load the projectile and propellant into the chamber of a gun/cannon than to reach all the way over to the front end to load ammunition and then push them back down a long tube – especially when the projectile fits tightly and the tube has spiral ridges from rifling. In field artillery, the advantages were similar – crews no longer had to get in front of the gun and force things down a long barrel with a ramrod, and the shot could now tightly fit the bore, increasing accuracy. It also made it easier to load a previously fired weapon with a fouled barrel. Gun turrets and emplacements for breechloaders can be smaller, since crews don't need to retract the gun for frontal loading.
After breech-loading became common, it also became common practice to fit recoil systems onto field guns to prevent the recoil from rolling the carriage back with every shot and ruining the aim. That provided faster firing times, but is not directly related to whether the gun is breech loading or not. Now that guns were able to fire without recoiling, the crew was able to remain grouped closely around the gun, ready to load and put final touches on the aim, subsequent to firing the next shot. That led to the development of an armored shield fitted to the carriage of the gun, to help shield the crew from long range area or sniper fire from the new, high-velocity, long-range rifles, or even machine guns.