What is meant by "functional" theme?

Hey. I’m new to themeforest and would like to start using this platform to sell my own themes. And I would like to clarify: what is meant by “functionality of the theme”? These are your own shortcodes, is it a framework? These are theme settings in Wordpress customizer(and if so, what settings should be there? ) Tell me, what should I do and connect to my theme to make it “functional” for reviewers and buyers of themeforest?