OPDS is an open standard for browsing and consuming content on your server from external readers. To enable OPDS for your server, under Server Settings > General, enable OPDS.
This will grant all your users a unique URL and API key (like a unique password) to access your server from external readers. The URL can be found under user settings.
From this UI, a user can use the copy button to quickly copy it into their clipboard. The red refresh button is used to generate a new API key and thus a new URL.
Kavita supports OPDS-PS (page streaming) which will capture your progress when reading through external readers. Currently, Chunky Reader and Panels on iOS supports OPDS-PS. OPDS-PS is only supported for image based media.