This test checks to see whether the website is W3C compliant.
W3C is the standards body for the web, and W3C compliance is therefore a widely adopted test with many benefits. As a general rule, sites which are W3C complaint will work more consistently across web browsers, in particular new ones as they are released. They will also likely be more accessible and usable (though adoption of standards which users will tend to expect, and therefore be familiar with). They are also more likely to work on non-standard devices, such as mobile phones and televisions, which are becoming increasingly prevalent. Some search engines (notably MSN and Google Accessible) prefer W3C compliant pages.
The principles of validation generally correlates highly with good SEO (Search Engine Optimisation) practice. W3C compliance is considered one of the best assurances that the website has been built with accessibility in mind (which is a legal requirement in many countries). The UK is particularly concerned with W3C compliance.
For more reasons see: http://validator.w3.org/docs/why.html
The W3C test is run on every page we test in turn. The overall score is based on the proportion of pages which contain no errors - a perfect score means 100% of pages are compliant.
Although you may get errors and warnings, only Errors count towards the final score.
Use the results of the validation to fix and improve the compliance of your website. For a new site, try and make the website W3C compliant from the start as it is hard to fix a large site which hasn't been built with W3C compliance in mind.