It's probably impossible to explain monstrous moonshine in simple terms, and I am not an expert in the areas of math related to monstrous moonshine, but here's a stab at it. read more
It is now known that lying behind monstrous moonshine is a vertex operator algebra called the moonshine module (or monster vertex algebra) constructed by Igor Frenkel, James Lepowsky, and Arne Meurman in 1988, having the monster group as symmetries. read more