The YouTube Shorts Library is a searchable collection of tracks that users can choose from to add music to their Shorts videos.
When you opt into eMastered’s Social Media Monetization Release Extra, your release is added to this library.
That means your music becomes available for use in Shorts, and you earn money each time it's used in a Short.
To learn more about how YouTube Shorts work, check out the official info from Google/YouTube.