A mush pot is a cast iron vessel that lead is melted in. Plumbers use them as well reloaders that cast their own bullets. The pot is kept at a temperture that keeps the lead in a mushy state, but not molten. After you heated and tinned the area you want to work, you pick up some of the softened lead with the back of your paddle, and apply it as if you were putting plaster on a wall. This method is so much easier when you have a large area to work.
I forgot to mention that the bars are cut lenghtwise for hand application.