Your EPUB can define the beginning of its main content. Reading systems can use this setting when a reader first opens your book, to take the reader straight to the main content.
In an EPUB 2 file, the start location is set in the OPF file, in the <guide> element. It looks like this:
<reference type="text" title="Startup Page" href="chapter1.xhtml"></reference>
For a full example of the <guide>, see Adding a <guide> to your OPF file.
In an EPUB 3 file, the start location goes in the landmarks nav in your Navigation Document. It looks like this:
<li><a epub:type="bodymatter" href="chapter1.xhtml">Start of Content</a></li>
For a full example of the landmarks nav, see Adding landmarks to your EPUB 3 file.