Low grade Marijuana, also know as dirt. It is so low grade and old and dry that it is brown instead of green. The only way to get any kind off a buzz out of it would be, to smoke half an ounce or more, non stop out of a bong, but since nobody wants to taint their bong with something so nasty, it ends up hand rolled in a flavored blunt to add a hint of flavor and to get a buzz off of the tobacco because that dirt aint getting any real stoner high for more than 30 seconds by itself. The reason it is called the chocolate stuff is that some potheads like to have names for their grass, and even a shitty batch needs a name to make it interesting especially to pot smokers who only normally smoke high grade hydroponic weed, and have to be convinced to try something that may have been green in 10,000 B.C. but currently looks like a tumbleweed mixed with chocolate cake batter and crushed pine cones.
Carlos: Did Justin have the Greens?
Ralph: Nah, I had to go to Fat Pat and get the chocolate stuff.
Carlos: I aint putting that shit in my pyrex
Ralph: Nah, we're gonna take the chocolate stuff and put it in the vanilla thing.