in your include page, only have the main part of the code. you don't want <html><head><body> tags because then when you include it into your main page and view source, it will say that there are 2 sets of <html><head><body> tags, because in this case there would be those tags in the main page and the menu page.
main page example:
<html>
<body>
<?php include 'menu.php'; ?>
hello world!
menu page example:
<ul>
<li>menu item</li>
<li>menu item</li>
<li>menu item</li>
</ul>
bad menu example:
<html>
<body>
<ul>
<li>menu item</li>
<li>menu item</li>
<li>menu item</li>
</ul>
if you included the bad menu example, as you can see, there would be two sets of html/body tags, and this is what he was talking about. hope that's clear lol.