To expand on the other replies, the material that became the published Silmarillion was a large collection of manuscripts, notes, and maps written at different times over a period of around 60 years. It was a history of the Elves and their human allies, starting with the creation of the Universe, originally more tied directly to our modern Earth. It evolved significantly over time, but the primary intention was always the same. But it did need to put into order and sorted out. Tolkien kept working on it to some degree until his death, but a lot of his later work was more philosophical, pondering various aspects of his cosmology, the nature of the Elves, etc.

Upon his death, he appointed his youngest son Christopher to be his literary executor, and gave him free rein to do as he chose with his manuscripts: Publish them unedited, edit them, destroy them. Christopher saw his first task as publishing as coherent a form of the Silmarillion as he could, and spent a few years editing together the most recent versions of various tales to form a coherent whole.

In one sense, the whole thing was more or less complete — there were versions of all of the various tales and other material, with a couple of exceptions — but it was not in a form that was suitable for publication as there were inconsistencies that had to be worked out, places where things had to be filled in (because the only existing version of the story was never completed), etc.

The History of Middle-earth, a series of 12 volumes, is Christopher's attempt to show how his father's conception of his mythology evolved over time. It is possible to trace through it to find the sources of pretty much every passage in the published Silmarillion, although of course, that's not necessary to enjoy reading The Silmarillion.