A static URL is one that does not change, so it typically does not contain any URL parameters.
Updating these kinds of pages can be time-consuming, especially if the amount of information grows quickly since every single page has to be hard-coded. This is why webmasters who deal with large, frequently updated sites like online shops, forum communities, blogs or content management systems may use dynamic URLs.
If the content of a site is stored in a database and pulled for display on pages on demand, dynamic URLs may be used. In that case, the site serves basically as a template for the content.
Dynamic URLs have the disadvantage that different URLs can have the same content. So different users might link to URLs with different parameters which have the same content. That's one reason why webmasters sometimes want to rewrite their URLs to static ones.