The Submit section allows your users to send you content directly from the app. It can be text, photos or videos. This content will arrive directly in your back office.
To add a Submission section, as for all the other sections of your app, you just have to go to the My app > Sections menu and click on Add a section. Then choose the Submit section.
There’s nothing more to do to set up your section. Unless you only want to offer one or two types of content. For that you will have to add the API Access add-on and edit the JSON of your section. Don't panic, this is very simple, and step by step online help is available in your back office.