I like Apple's description:

> A URL scheme is the part of a link that **specifies the type of application your device uses to open a URL**. Many apps support URL schemes: FaceTime uses URL schemes to make calls when a URL starting with `facetime://` is opened, just like Safari handles URLs starting with `http://`.
>
> Shortcut’s URL scheme, `shortcuts://`, launches the Shortcuts app from a link to perform a task such as importing or running a shortcut. These links can be useful in many different contexts such as on the web or when using Shortcuts with other apps that support opening URLs

[Source](https://support.apple.com/en-au/guide/shortcuts/about-url-schemes--apd621a1ad7a/ios)