A better way of putting it.. each layer of a song separated from each other. Producers usually share them with each other for making official remixes. Example...
Stem 1 - Lead vocal
Stem 2 - Background vocal
Stem 3 - Bass line instrumental only
Stem 4 - Piano instrumental only
Stem 5 - Violin instrumental only
Individually they are only one sound, but layered on top of each other they make the complete song. Of course when you have the stems you can rearrange all the elements and create a total different record. No one does that anymore though. This generation just adds a rap verse and call its a remix ?