The M's stand for the masses of the two objects. The "d" stands for the distance between the objects.
The following link is a quick bite from the pancreas song. It's not high quality, I had to compress it, but it shouldn't be TOO painful to listen to (dial-up friendly).
Notice something? Did Weird Al actually work in the equation for how his pancreas is gravitationally attracted to every other pancreas? Does this seem weird to anybody else but me?