What are rewrite rules?

A module on an Apache Web server that can be used to manipulate the data passed in to a Web URL, for example redirecting the user to a PHP page with various variables being passed in at the same time.

The page you are looking at now uses rewrite rules to pass the selected Glossary Term to a PHP page which then gets the information for that term from a data file.

The upshot is it means the website can have more Google Friendly URL's, eg

http://www.nigelbunner.co.uk
/glossary_Rewrite-Rules.html

instead of

http://www.nigelbunner.co.uk
/glossary_page.php?search_term=Rewrite-Rules