Well "SEO" is the art of Search Engine Optimization, (optimizing your website so that it ranks better in search engines), so vBSEO is an SEO hack for vB. It makes it easier for search engines to find and index the valueable content inside your forums, so that people searching for a particular topic that you cover are more likely to find your site amongst their search results.
Is there a FREE versin of this? 149 bucks seems STEEP!
You get what you pay for. You want something that correctly changes your urls you get VBSEO. You want something that is supported you get VBSEO. You want something that is being developed constantly you get VBSEO.
You get the idea.
There are free hacks for rewriting urls. However, you get what you pay for.