In terms of what this means for users, there's not a lot of difference -- you'll still click the icon on your homescreen and the web app will launch like normal. But behind the scenes, if the web page you load uses Nitro, it'll work quicker than usual. Keep in mind that neither full-screen web apps nor browser views within apps have had access to this high-speed engine in iOS 4, so this is an improvement.
But it's nice to know that some of the web apps you may use on your iPhone or iPad may see a speed boost with the new OS. When it comes to web pages, faster is almost always better.