From the MIDI spec:
A sequencer’s Song Position (SP) is the number of MIDI beats (1 beat = 6 MIDI clocks) that have elapsed from the start of the song and is used to begin playback of a sequence from a position other than the beginning of a song. It is normally set to 0 when the START button is pressed to start sequence playback from the very beginning. It is incremented every sixth MIDI clock until STOP is pressed. If CONTINUE is pressed, it continues to increment from its current value.
The current Song Position can be communicated via the Song Position Pointer message and can be changed in a receiver by an incoming Song Position Pointer message. The message should only be recognized if the receiver is set to MIDI sync (external) mode.