To understand what HTML 5 is we have to first understand what HTML is. HTML is markup language for describing web documents. With HTML, different tags are used to describe different documents. Here’s an example of HTML from W3schools
HTML 5 is the 5th iteration of HTML. As time goes on, HTML standards and abilities are changed, errors and compatibility issues fixed, and features become more robust. Looking at the multimedia functions of HTML 5, you find your biggest change, is stepping away from Flash Player usage. With HTML 5 multimedia plugins load by themselves. HTML 5 also provides more support and functionality for various mobile devices. For further information on HTML 5 feel free to check out this helpful article I found here at Tech Target.