YouTube uses a system called Content ID to fingerprint music and allow the owners of the music to track and/or monetize the use of their music on YouTube. When YouTube recognizes a song that is being used in a video it alerts the uploader that the video contains music owned by a “Third Party” (that would be us in this case).
It does not mean you are receiving a copyright infringement notice or that a strike is being placed on your channel. It also won’t cause your video to be taken down. It does, however, restrict you from monetizing the video yourself on YouTube.