This is the position at which the song starts. It determines the Song view's left margin, and the Rewind command's destination. It's displayed either in beats and ticks, or in measures, beats and ticks, depending on the Meter property.
Negative start positions are allowed, and this may be useful because a full convergence is guaranteed at position zero (1:000). By starting at a negative time, you effectively move this initial convergence to later in your song.