If you want to gain a basic understanding of what an EPUB file consists of, and how the different bits fit together, there's a pretty good EPUB tutorial on jedisaber.com :
http://www.jedisaber.com/eBooks/tutorial.asp
Another good tutorial, though much more detailed and a lot more technical, is available in the IBM technical library :
http://www.ibm.com/developerworks/xm...tut/index.html
Note, though, that if you're using Sigil, it normally shouldn't be necessary to go in and edit the metadata files inside the EPUB file directly, as Sigil handles that for you, enabling you to focus on the book content. But understanding what goes into an EPUB file might make it easier to understand what's going on beneath the surface, and easier to understand how best to make use of the the tools provided by applications like Sigil. It can also help when trying to figure out what is causing specific errors with epubcheck and similar tools.