Look up carbon to nitrogen ratio. ( don't be distracted by all the info on how it relates to global warming, wait, there IS NONE) sorry couldn't help it. Carbon is just referring to any waste material that's low in nitrogen, straw, wood chips, dried dead brown grass. Nitrogen is higher in higher protein wastes, chicken manure, green grass clippings etc. High nitrogen waste will get nasty, so it has to be mixed with brown stuff to balance the carbon to nitrogen. Capping is just putting a cap of less attractive waste over waste that will attract pests.