A more thorough explanation:

An underground recording, also known as a bootleg recording, is an unauthorized copy of a live or broadcast performance that is made available to the public without the permission of the original artist or copyright holder. These recordings are often distributed through illegal channels, such as online file-sharing sites or physical copies sold on the black market.

• A fan records a concert on their phone and shares the audio online without the artist's permission.
• A radio station broadcasts a live performance and someone records it off the air and distributes it without permission.
• A person sells unauthorized copies of a popular album on the street corner.

These examples illustrate how underground recordings can be made and distributed without the knowledge or consent of the original artist or copyright holder. This can result in lost revenue for the artist and can also lead to legal consequences for those involved in the distribution of the recordings.