A master is the original recording of a song
A master is the original recording of the song or composition.
An example of a master is Taylor Swift’s original recording of her 2008 album Fearless under Big Machine Records. Swift, who owns the rights to the composition, but not the master, re-recorded the album in 2021 with another record label, Republic Records. Her deal with Republic ensures that she retains not only the composition rights but also rights to the masters.
She did this to replace the original recording with her new one, and divert the revenue from these songs to her, instead of her nemesis Scooter Braun, who bought the masters of her albums from Big Machine in 2018. Braun recently sold her masters to an unknown investment fund for over $300 million.