The Stuff is a monster from the 1985 film of the same name.
It was unearthed by miners by accident. When tasted, it was proclaimed so good it had to be packaged and sold in stores. The Stuff soon proved addictive, and dangerous - eating enough of it causes the unfortunate addict to become zombielike, with Stuff for blood. It was outlawed at the end of the film (which didn't stop the black market from selling it), and the Stuff that was left over was force-fed to two moguls who profited from it.