This passage is basically the first part of the creation myth that Tolkien designed for the world he wrote about. It tells about how Ilúvatar sang the Ainur into being and how the world and elements became real. The story mainly deals with Ilúvatar's vision and design of the world - through music.
The first part of the chapter "The Music of the Ainur" consists of a text which picks up from where the "Cottage of Lost Play" ended. It mainly describes a meeting between Eriol and aged Rúmil who dwells at the cottage and cares for the garden. This part forms a link between the "Cottage of Lost Play" and the tale of "The Music of the Ainur".
"The Cottage of Lost Play" is the first chapter in The Book of Lost Tales part 1. It forms a framework narrative in which the other tales of the two books are told. The chapter features Eriol, a mariner, who finds himself on an island where he encounter elves who tell him tales of their history and the world .
The Book of Lost Tales is the first two books of the 12 volume collection of The History of Middle Earth. This is a compilation of material written by J.R.R. Tolkien put together by Christopher Tolkien.