I'll try to explain it the best I can without cracking useless jokes. In this mythos there exists a diety called Yog-Sothoth, that watches aover all of creation, being omnipresent, reason for which he answers easily when he is called upon, usually to bestow knowledge in exchange for a price. And old man in Dunwitch summons him to impregnate his granddaughter, which Yog agrees to and thus 2 sons are born. One of them ,Wilbur, is born disfigured, goatish in appearence, and after a point he is the one that attempted to steal the Necronomicon in order to fufill his grandfather's end of the bargain and open a gate to allow the Great Old Ones to return into this world (they were beings of incredible power, calling them gods would barely be fitting to represent them, though they are far weaker when compared to Yog, they were locked away aeons ago and entered slumbers with some still atempting to rise again) but he fails before he can complete the ritual and bring them forth.The other child, though, resembled his father more closely, he was an entity that humans couldn't see, without a defined physical form, that fed on cows and other living creatures to increase its power, some say that once it gained enough might he would set the Great Old Ones free himself, alas, through the Necronomicon, he is sealed away as well. It is unclear why Yog would want the return of the Great Old Ones and why he couldn't do it himself, though some say his actual influence is limited. The Necronomicon is a book written by a man that had a certain insight into these matters, again, it is unknown how he gained this knowledge, maybe it was given to him by Yog, or Nyarlathotep, or a Great Old One,.
This isn't a clearly defined mythology like the one created by Tolkein. This one is glimpsed at through the eyes of humans, that were never meant to fully comprehand the truth.