Oh, so there's a fantasy series called Lord of the Rings (I'm only saying this because people come from different words, who knows what's translated, if you know this just disregard) about a journey to destroy a ring and save the world from the Dark Lord. The Silmarillion is a collection of stories about the history of that world, and Children of Húrin is the expanded story of one of those stories.
Basically like, the history of the history of the main story. Not what I'd start people on if they were interested in the series, because even stalwart fans are divided on it, but I like it.