HTML5 has become quite the buzz word lately, and it's important for people in both developer roles and project management roles to understand what it really is. The term has been around long enough that clients are aware of it, and may begin to ask "so how will you incorporate HTML5 into our website?" Not knowing how to answer this could be disastrous when attempting to gain the trust of a potential client.
So what is HTML5 then? For the most part, HTML5 adds more tools to the toolbox for developers. I feel the most important addition of HTML5 are the new semantic markup elements. For years and years, developers have been using <div> elements to define the structure of their website (and before that, horrendous tables). A problem with doing this is that there is no true semantic value in throwing hundreds of div tags on a website. How does Google know where your site's main navigation is? How about where the caption for a picture is? Is this page a listing of a bunch of news items? HTML5 introduces really helpful elements like <nav>, <header>, and <article> that help enrich the semantic value of a web page, which will eventually provide SEO benefits.
In addition, there are the new <video> and <audio> tags which help serve multimedia content across multiple platforms (especially mobile) in a very easy way. The controls for the player can be easily styled using CSS, and developers no longer have to worry about getting a decent flash player, customizing it, and making sure there's a fallback for mobile devices.
There are some other nice enhancements that come along with HTML5, but these mostly benefit developers and shouldn't be a concern of clients or managment.
What you should know as a project manager:
For the curious, here's a nice list of the major features of HTML5: http://net.tutsplus.com/tutorials/html-css-techniques/25-html5-features-tips-and-techniques-you-must-know/
And here is a great article that goes much more into detail of what HTML5 really is: http://radar.oreilly.com/2011/07/what-is-html5.html