Question

What is a bootleg music? Is it a concert that has been factory pressed, and also,
not been officially released? Are non-copyrighted concerts legal to download?

Bootleg music is a recording of a an artist's performance that has not been officially released either by the artist concerned or a record company.

Fans of the artist concerned often swap bootleg recordings with each other and rare recordings can be enhanced by professional track-recording techniques before being packaged and sold on.

However, copyright of the material and the right to record the material normally belong to the artist concerned, so I would imagine that according to the letter of the law, there is no such thing as a non-copyright concert and downloading it would be illegal.

Some musicians actually encourage bootlegging because it raises their profile within the live music fraternity and can encourage fans to go and buy their records.