A more thorough explanation:

A bootleg copy is an unauthorized copy of a live or broadcast performance that is made available in a tangible medium or digital duplication, often over the internet. It is also known as an underground recording, import recording, or pirate recording.

For example, if someone records a concert without permission and then sells copies of that recording, it is considered a bootleg copy. Similarly, if someone records a movie in a theater and then distributes copies of that recording, it is also a bootleg copy.

Bootleg copies are illegal because they violate copyright laws. Copyright laws protect the rights of the creators of original works, such as musicians, filmmakers, and authors, to control how their works are used and distributed. When someone makes a bootleg copy, they are essentially stealing the intellectual property of the creator and profiting from it without permission.