Custom URL Schemes are a way to register a URL with the OS, e.g. myapp://doSomething and have my app launch if they click it. They can even take the user to specific screens, which is more than I need, but would probably help others.
Currently I’m sending text messages to notify users of a new message. But they have to manually launch the app. If there was a URL, they could just tap that URL to launch it.
Here’s Apple’s docs on Custom URL Schemes: https://developer.apple.com/documentation/xcode/allowing_apps_and_websites_to_link_to_your_content/defining_a_custom_url_scheme_for_your_app