The Silmarillion is the collection of myths from the ancient days of Arda. Its central storyline is the war for the Silmarils, the precious gems made by the gifted Elf Fëanor and stolen by the evil deity Melkor/Morgoth. We are dragged into a world of its own, full of heroes and anti-heroes, grand and utterly detailed. We encounter love and loss, friendship and war, pain and grief, joy and hope. The depth of this vision can be both terryfing and joyful at once. Though in a way a prequel, the Silmarillion is best consumed after having read Lord of the Rings to save oneself from a massive overdose. However, once you dipped into it, you will soon come to understand many more of the very details behind the LOTR, such as the odd relationships between Elves, Men and Dwarves, or the role of Galadriel for example.
It is indeed hard to grasp how one mind could conveive these tales, this whole world rich in myths, consistent in itself. Attempts were made, yet never has the grandeur of Tolkien's world been challenged literarily. The regions of Arda are vast beyond the thoughts of Elves and Men!