What does gas blowback mean on a rifle?

gas blow back is when a bullet is fired the gasses propel it along the bore of the rifle. After the bullet leaves, some of the gas is expelled and some of it goes onto a second tube at the top. on the ak-47 it is the weird looking curved tube above the barrel. on an m16 it its the front aiming sight bar that rises up. and then goes into the handgaurd. Tej gases then shoot backpushing the empty cartrige out causing the machanism to slide another from the magazine in its place.