DNS stands for Domain Name Server, it is responsible for conversion of domain names like (google.com) into respective IP address. By default when you try to open website, ISPs (Internet Service provider) DNS server converts the domain name into IP address and allows you to open website. If your ISPs DNS is slow or over-crowded you can switch to 3rd party DNS providers like Open DNS for faster browsing.
Similarly, Google public DNS is a third party free DNS service (an alternative to your current ISPs DNS). Google already crawls majority of content on daily basis and in the process resolves DNS information to index content on different websites or blogs. Hence, it can use that to provide solid free DNS service alternative.
Google public DNS features
1. An alternative DNS service for more valid results, increased security, better performance (as Google continuously crawl, resolve and index new content every second on internet).
2. With its global data-center and caching infrastructure, Google DNS can serve large numbers of user requestswithout over-loading ISPs DNS server.
3. As of now, it is an experimental service aimed to leverage Google crawl and index capabilities to support DNS for faster internet.