Ideally, you should submit a Debian source package. A Debian source package consists of 3 files (with extensions .dsc, diff.gz, orig.tar.gz), which you should put in a compressed archive (a tarball, zip file, rar...) and upload into My Apps. This will allow reviewers to easily test and publish your app.

However, there are some important caveats:

If your app is commercial or proprietary software: we still recommend uploading a Debian source package, but if you are not experienced in packaging you can also upload either a binary Debian package (.deb), your source code in a compressed archive, your binary executables and all files needed to run your app in a compressed archive, and the commercial packagers will package and publish it for you.

If your app is Free Software and gratis: we recommend using a Personal Packaging Archive (PPA). You can specify the location of your PPA in the 'Any additional notes for the application reviewer' text box in the Overview tab of your app's entry in My Apps. You can also learn more about PPAs in the packaging section of the Ubuntu App Developer Site (*)

(*) Sorry for the confusion here, we've recently switched to recommending PPAs for Free Software gratis apps, and the web UI might be a it confusing.