Music transcription is the intricate process of transforming audio into sheet music that performers can read. A music transcriber will listen to the audio of a musical performance and write down what they hear in the form of music notation.
This is a tedious process that requires intimate knowledge of how music is written and performed, the capabilities/limitations of the instrument(s) for which the music is written, and trained ears with the ability to focus on individual instruments, pitches, and rhythms within a single audio file.