the "wall" refers to the bow at full draw at the point where you can't pull it back anymore. If you say it has a firm back wall it means once you hit full draw you can't pull it back anymore, it feels solid. If it feels alittle soft and stretchy when you pull into the back wall then it has a soft back wall.
"stops" is what stops the string and cams from being drew back any farther, it could be a peg that hits the limb or a rail in the cam that hits the string on the back side. Most of the time the stops control how the back wall feels.
Hopefully this helps.