Forking an app will make an exact copy of the app in a new environment. That means the new app will automatically configure itself to be identical to the source but it will have its own data, schema, and API. This is useful for testing schema changes before making any migrations to your production applications. Once you have tested your changes and manually merged them into your production app you can safely remove the fork.
You can fork an app in 2 ways:
Apps Page > Underneath each API link for each app panel (top-right)
Settings Page > Advanced