WebAPI is basically an idea from Mozilla which promises to bridge the gap between native and web applications.
This can be done by using, for example Phonegap (iOS, Android, Blackberry OS, WebOS, WP7, Symbian & Bada) which is quite popular, or write an app and put it under the Cocoa Touch iPhone App Wrapper which will make it a native app.
Now… Mozilla’s idea on bridging the gap between native apps and web applications is quite different from frameworks like the aforementioned Phonegap because the purpose of WebAPI is not merely to make native applications but to reach low-level functionality of a device using HTML5.
Of course, if Mozilla wants this to be a success, it has to be adopted by companies like Google, Apple and Microsoft. So Mozilla has to, first and foremost, convince users it is the way forward. For the time being they are hiring developers for it.