It's an alternative app store for open source apps - something sort of like extras, but for Harmattan.

In order to get started, sign up for a meego.com account, then get on IRC (Freenode #meego or #harmattan) and pester lbt or X-Fade to enable your account for Community OBS. Make sure you follow the packaging guidelines, then build a Debian source package out of your project and upload it to your OBS "home project". Once that's done, you can then submit into Apps for MeeGo. (If that sounded really complicated, it isn't actually all that bad)

The tricky part is building the source package, which Qt Creator won't do for you. phako wrote this script, which might be a good starting point - run it from a clean source tree and it should produce the needed package.