An apache module mod_rewrite is the technical term referring to an Apache module that's used by web developers to rewrite the URLs of sites that are hosted on an Apache server.
In the world of Search Engine Optimization (SEO), webmasters use an apache module mod_rewrite to solve certain problems that may arise. An apache module mod_rewrite is useful when trying to avoid duplicate content, redirect non existing inbound links or URLs that were moved, and create URLs that are simply more search engine friendly.
Say for example that your sites URL looks like:
Not the most attractive, easy-to-remember, easy-to-read URL in the whole world is it? With an apache module mod_rewrite, webmasters can retool this ugly, incomprehensible URL into something better not only for SEO purposes, but also for users, like:
Compare the before and after URL. Doesn't the second one look much more friendly, easy to remember, and easy to read?
Besides solving problems and doing damage control, an apache module mod_rewrite can also be used to enhance a site's search ranking, helping Google and other search engines to index it much faster so that it can start receiving more visitors faster.
Essentially, it's a powerful tool that webmaster can use to manipulate URLs.