PHP allows you to connect your html pages to many other things, like files, databases and even other webservers.
For example, you can use PHP and a database to store news articles, links to webpages, or any text you like, to create dynamic pages.
PHP can also be used to create navigation menu's in pages.
That way you can write on script that prints the menu, and call that script from all your pages, so the menu shows up in all pages.
If you want to change the menu, you just change the menu-script.
Next step could be to make the menu appear different for some pages.
This saves you a lot of html editing.