What is "Content ID"?

A brief summary on Content ID

Content ID a digital fingerprinting system developed by Google for Youtube. When content is uploaded to Youtube, Content ID checks against a database of both audio and visual fingerprints to check for matches. If a match is detected, the original content owner has the power to issue a content takedown or monetize the infringing upload. It's important that Pretzel.Rocks users have "Youtube Safe" toggled on if they intend to upload any part of their stream to Youtube. This is because Pretzel has ensured that any music in the "Youtube Safe" catalog has been granted full permission for use on Youtube, without democratization or takedown notice.

You can read more about "Youtube Safe" here