We use the word offal to refer to the parts of a butchered animal that aren’t going to be eaten. These are the pieces that “fall off” a butcher’s work counter. In Shakespeare’s time, offal could also refer to the chips and shavings that fall off of a carpenter’s work bench, and that’s how Cassius is using it here. He seems to be saying that Caesar uses the citizens of Rome like kindling to start a fire that will brighten his own image, just as the carpenter uses his offal to build a fire to illuminate his workshop.