Some of the most watched and listened to music on YouTube and YouTube Music disappeared on Saturday, replaced by a short message:

Video unavailable

This video contains content from SESAC. It is not available in your country.

That is probably not what people expected when they tried to stream Adele’s “Rolling in the Deep” or other songs from artists as varied as Kendrick Lamar, Britney Spears, Green Day, Kanye West, and Burna Boy.

This leaves two questions: what is SESAC, and when is the…