Trackouts also known as tracked out audio files, multi-tracks, stems are the separate high-quality WAV audio files of an instrumental, for example Kick, Bass, Snare, Hat, Piano, Percussion, Instruments, FXs etc. The reason for stems is that they allow for more precise fine tuning while mixing a song so you or your audio engineer can get a proper mix matching the vocal recordings. In other words track-outs are very useful for high-end mixing projects where you or your audio engineer may need to customize the mix of the instrumental around your vocals, for example if you need to add/remove any sounds of the beat, to make the beat or any part of it longer/shorter, to change the structure of the verse or chorus, to change the leveling of the sounds (for example to make the bass louder), to add/cut frequencies in any particular sound of the beat etc. This way, you or your audio engineer will have more control over editing, mixing and mastering the beat and the song.