It is a web design standard set by the w3.org website which is known and used worldwide. You can check the code behind your website to see if it's suitable or not for this standard.
W3 is short form of World Wide Web. In an unofficial capacity it is first web site of Internet. It helps in determines standards used on the Internet. The founder of W3C is Tim Berners Lee. He is a computer scientist and provides consultancy to many companies, such as Google. W3’s official website is at www.w3.org. You can find the html, css errors on your website by using this site and can correct them. If you make your website according to W3 standards and you control over W3 and you didn’t face with any problem, your web site was made according to the W3 standards. The standards can in raising your place on a SERP (Search Engine Results Page) on Google. You can also make donations to the W3C. If there are no errors on your web site, you can show this success in the form of a badge on your website.