Remember, the Silmarillion is not a story. It is the collected mythology of Middle-Earth. It is a series of legends, heroic epics, and creation myths, which serves as the backstory for the LotR trilogy and The Hobbit. It cannot be summarized, except in the most basic of forms, IE one could refer to it as the Bible of Middle Earth. Like all summaries, this one is mostly inaccurate.
The book has two main themes: the betrayal of Morkoth/Sauron, and the triumph of good over evil. If you want to know where the races of middle-earth came from, this is your book. If you want to know more about Sauron's origins, this is your book. If you're looking for a story in the tradition of The Hobbit or LotR, look elsewhere.
Here you will find tragic tales like the story of Beren and Luthien, man and elf-woman. You will read of epic battles, and hear of Feanor's vendetta against Morkoth, and learn of Telperion, the tree of which Galadriel speaks when she gives Frodo the vial.
A summary will do little but whet one's appetite for the book, if well-written. In that sense, I would encourage it. But I'd just as soon say, 'You'd have to read it to understand. Go buy a copy.'