Not to be confused with Soylent Green, Green Stuff is a Games Workshop product intended for miniature conversion. It is a two-part epoxy putty; the user kneads the two components (yellow and blue) and then molds the resulting putty (green, hence the name) into the desired shape.
Of course, GW being GW, Green Stuff is incredibly expensive; lots of companies make two-part epoxy putties (which are often colloquially called "green stuff" because they serve the same function and are green) for less.
With the advent of Finecast, Games Workshop realized that they were going to have lots of bad molds. Rather than fixing the molding issues, they decided to make even more money by selling tools so the schmucks who bought flawed models could fix them themselves. This includes Liquid Green Stuff, Green Stuff that has been thinned so that it can be painted into gaps.