Includes will "include" another item where it is specified. The sample below is a PHP include. HTML includes work the same way. They are useful to use when they need to be inserted into several pages without Cut & Pasting the document itself into the page. When you change the Include, it will update any page using that include.
Samples of a PHP Includes
The Include to Import
Result
<?php include ("../../includes/training/scriptsDivCss.php"); ?>