Custom Content gives you the option to add extra content to the top of the first page of an app. It can be used to welcome users, or give clarification to an app's purpose.
This feature should be used sparingly. Everything in Twine is designed to be self-explanatory and only in exceptional cases should custom content be employed at the top of pages.
Creating Custom Content is straightforward. Head to Settings > Custom Content and use Key to select the page you want to add custom content to.