HTML is what your web browser displays in front of you. You're looking at HTML right now! It's what makes web pages. HTML5 is being developed as the next major revision of HTML (HyperText Markup Language), the core markup language of the World Wide Web. Google apps will require that you have a web browser that can output HTML5. Google Docs already refuse to work with anything less -- and before long that will apply to the entirety of Google Apps. August 1 is the set deadline. After that your user experience will deteriate unless you're using either the current or last major release of Internet Explorer, Firefox, Chrome or Safari.
Time to upgrade your browser.