In essence, HTML5 is a cross-platform programming language for multipurpose web application development. It lacks the performance "bugginess" of Adobe's fading Flash format, and is beginning to impact software development, with many choosing to deploy it within their applications, even within conventional desktop apps. More importantly, because this is an HTML language, apps built using it run well on all manner of devices -- a major plus for enterprise users in the BYOD transition.
A recent identified 75% who are using or plan to use HTML5 within their apps. This follows Adobe's recent declaration that Flex is no longer the best tech for creating and deploying rich content across mobile platforms. That, of course, is something once by Apple CEO, Steve Jobs.
the technology isn't yet ready for prime time use within business applications. Yes, it can be an element within such software, but work remains to be done to ensure rock-solid security and effective synch between devices. That work proceeds, with the World Wide Web Consortium promising a final definition of HTML 5 in a few years.
This doesn't mean you can't implement HTML5 within part of your application development -- we're in an age of transition, with hybrid solutions seen as the best way forward.