A "swamped barrel" is an octagon that starts fairly large at the breech, gets thinner as you move towards the muzzle and then gets bigger at the muzzle.
Because of this, swamped barrels are much lighter weight than a straight octagon barrel. This makes the gun feel much less "muzzle heavy".
Because the size of the outside is constantly changing people can't talk about it being one size like a 7/8" or 15/16" barrel.
To get around this, the barrel makers use letters to designate the size. An "A" size is the smallest or thinnest. A "D" size is the thickest.
"B" size barrels are quite light.
