Compiling is accessed via File -> Dev Options -> Complie (Alt+C).

Compiling runs the layout files through Handlebars for each entry in each group you've indicated you'd like to compile. By default, all groups are compiled, but only new entries are compiled. Compiling generates a layout document that is stored in the database, and contains the final HTML, CSS, and JS code for an entry's preview and view variants, in each of your game package's supported languages.