The Publish mode is an export mode that automatically converts the inline CSS styles of the UI elements in the scene to CSS classes. This can be very useful if you have external CSS styles that you want to have higher priority than the default inline styles.
To publish a file select the "Publish" option in the "File" menu and choose a name for the file. This is the file that needs to be used in the GT/SG/Hummingbird view in the game/application.
Editing published files
When you publish a new HTML file is created. So there are two files for the scene - a work file (the original file that has been created) and the published file (with CSS styles converted to classes). The published file has a reference to the work file so when you try to open the published file the work file is opened automatically instead. This way you can edit only one file. When you finish editing the file you can re-publish it from the "File/Publish" menu.