What is a song’s key?
In essence the Key of a particular song is the note or group of notes that the music is centred on or built from, basically the same thing as the root-note. To simplify, you could say the Key is the “main note” in a song (e.g. F#, G or Bb).
In DJing, knowledge of the key of a particular song is vital when attempting to mix harmonically – known as harmonic mixing.