Rewrite, also referred to as URL rewrite, is a technique to improve a web page's ability to be indexed by major search engines. For example, a web site's product page (let's use stainless steel toasters, as an example) with a URL like http://www.yourwebsite.com/page3.html might be rewritten using relevant keyword-rich text, to appear like http://www.yourwebsite.com/stainless-steel-toasters.html

Depending on the type of server that you are using there are various types of URL re-writing tools that may be available.

It is important that when selecting a content management system (CMS) that it has the ability to rewrite URLs for search engine friendliness.  The ability to incorporate keywords into URLs can help boost natural (organic) search engine rankings.