The Hobbit is basically a middle grade chapter book and the lord of the rings series is a more mature sequel to the hobbit. More appropriate for teens or adults reading for fun in its difficulty. Both of these things were made to be published and they take place in the same fictional world that Tolkien created.
The Silmarillion, on the other hand, was not actually meant to be published. It’s a collection of world building notes, myths, and fake history that Tolkien left in his papers that his son edited together after he died. It’s an incredibly dense book of basically nothing but lore for the universe that lotr and the hobbit takes place in.
Think the difference between a biography of Benjamin Franklin aimed at middle schoolers and a semester long college level course about the American Revolutionary War.
