Oh, well, here it goes, I guess.
First off, you need to know a little something about his author, one H. P. Lovecraft. Guy was freaking brilliant at writing horror stories. To the point where almost any horror author worth their salt was influenced by him. He was capable of utilizing his overly-florid writing style to the best of its ability, something that few authors can truthfully claim to do. He was also a notorious racist and held some antisemitic beliefs, but that's neither here nor there.
He basically created an entire mythos full of fictional deities, gods from outer space, and so on. (The "Yog-Sothoth Cycle", as he called it; the "Cthulhu Mythos" as everyone else called it) Yog-Sothoth was probably one of the most powerful of these deities, if not the most powerful (and wise). Yog-Sothoth was, in essence, all of space-time with more tentacles. It/he/she's in every point in space and time at once.
Or, better yet, check out it's Wikipedia article. Yeah, it has a ****ing Wikipedia article. That's should give you an idea about how influential Lovecraft is.