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

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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