Server Side Includes (SSI) is a simple server-side scripting language used in website. It is used to includes the contents of one file into another one dynamically.
You can use ssi by creating a file (left.ssi), and include the file where ever you want by using the code:
<?php include(‘ssi/left.ssi’); ?>
This will include the left.ssi located in the ssi folder. Why should you use SSI? Because it will save you a lot of time. If you have a code that will be used in many pages, then you need to use SSI. For example: your navivation should have the same code for all of your pages. Your index.php and your product page (product.php) will have the same navigation (menu) code. So you need to create a ssi file for your navigation code. Then include the ssi file in all of your files when needed.
If someday you need to add one menu, you don’t need to change all pages code. You only need to change the ssi code, and changes will be reflected to all pages. You should be using ssi file for your header, footer, adsense, ads place and navigation.