A split track is a track that puts all orchestration (music) on the left channel, and all voice on the right channel (or vice versa). If you listen to a split track in stereo, you hear both channels and it sounds like regular listening music. If you want to sing with it, you split out the voice channel with your left/right balance knob on the CD player, main receiver, or mixing board, as applicable.
Split tracks are frequently used for musicals and choral collections, which a church choir would sing. Since you don’t have to phase all of the voice channel out, it allows small groups or choirs to have a fuller range of sound, especially if they are missing key voice ranges.