Whenever more than one device - software, hardware, computer etc. - are used simultaneously to create one final thing - e.g. a movie with music, or a lightshow running in sync to music or to a video - then a way to synchronize all devices is required. SMPTE Timecode is something like the standard for such a task: the timecode master sends a continuous stream of data which permanently states the time the master has advanced into the show/track/clip. And all other devices - here: timecode slaves - are to react to the very time as programmed in each device.