SEO Audit means - Find out why your site is not ranking for your target keywords for e.g, $500 and how you can increase your traffic, conversion rates and sales?
a lot of companies give these out for free as an incentive to potetential clients, we do something similar, we have this thing called ten tips I can implement in 10 minutes, it's free and as you can guess we crawl your site, find ten problems and tell you how to fix them in you guessed it 10 minutes PM me the site and I'll do it for you.
When a company checks over your site and tells you what needs sorting out to optimise it for the search engines
audit as the term suggest itself. So SEO audit means auditing your websites for higher SERPs by indication al flaws and giving tips to increase SERP
Here is an article you will surely be interested in to know about SEO Audit. http://searchenginewatch.com/3641521