When you read Lord of the Rings there are innumerable references, some of them too puzzling and important to ignore, to events from the past; people, battles, places, names. It occurs in The Hobbit as well, but to a much lesser degree.
If you read either of those and don't like them, or found them hard going, so be it, and don't bother with this. But, if you read them and like them, you will be wanting to know what it was all about. Where did dragons come from? Who are Elrond and Gandalf? What is the Balrog? Where did Sauron come from? Who are the Men of Westernesse? What was the Last Alliance? Where did the One Ring come from?
Tolkien did that deliberately. He created a complete world, with a history from start to finish. Lord of the Rings is only the end of the tale that starts in the Silmarillion, with the beginning of Middle Earth. He wanted LotR to be the story, the compelling tale, but what happens in it, and the places it happens in, is all part of a great history.
That history is told in this book. From creation of Middle Earth to beyond the end of LotR, it covers everything that happened. It genuinely is a complete mythology.
For that reason, the Silmarillion is an inferior *story* to LotR, but tells you the *history*. It doesn't read like a story at all, but like a history, a bible of Middle Earth. Many, many happenings, places and especially names, will put off the casual reader, and rightly so. It is the stuff of legend, too dense for anyone without an interest in getting to the heart of Middle Earth and, therefore, LotR and the Hobbit.
One of the best things is the glossary in the back, which has in it every term, name, thing and place in Middle Earth, and what they are. Of course, even though it is technically a prequel to LotR, don't attempt reading it first. Like I say, it fills out the world that LotR and the Hobbit create.