What are metadata descriptions in a website?

1. Metadata: these are fields containing information for the browser and search engines. Good metadata is important to make the site "visible" by search engines. Their "contents" is analysed by SE to classify your site.
For more information, google "html metadata" (the list is huge!)
2. Validating: a antediluvian technique to "validate" a piece of HTML code, nowadays 90% useless, since many features are not recognised by the validator, and fail. (NO "big" site will pass validation!)
Can be useful for beginners to learn the basics of HTML coding, but it stops there.
When a "developer" affixes, proudly, the w3validation logo on his site, I KNOW the site is crap and contains nothing interesting... And when a client demands the validation, I KNOW he knows nothing about the web...(and I tell him to go away) :-)