This stands for the World Wide Web Consortium which is the international governing body for the web. Tim Berners-Lee, the director of the W3C, is also one of the original inventors of the World Wide Web. W3C’s primary activity is to develop protocols and guidelines that ensure long-term growth for the Web. W3C’s standards define key parts of what makes the World Wide Web work.
Keep in mind there are web standards much of which relate to accessibility and some developers are better than others writing code that complies with standards.