This is about creating an open standard for APIs a phone OS would need to implement so that pretty much any app people have on phones right now can be implemented with web technologies (HTML, CSS, JavaScript). Essentially, any web page can be an app; apps that need access to some sort of hardware capabilities that web pages don't normally have access to would need to have a permissions system of some sort, of course.

It's also about then shipping actual phones with an OS implementing these APIs installed, creating an "app store" to aid in app discovery and whatnot and whatever other pieces of infrastructure are needed to actually get this open standard used.

One long-term goal, of course, is being able to switch phones (or carriers, or phone OSes; whatever choice the consumer wants to make) without losing all your apps; hence the need for an open standard.