Sync refers to the ability to integrate one environment with another such that the both perform and record at the same time. To sync two systems, there must be one master that provides timing information, and one or more slaves that follow the master time. Currently Temper can only be a master, and only via a MIDI port.
To slave another environment to Temper, first create a new MIDI track with an output port connected to another environment, then set the appropriate sync settings. Full descriptions along with tutorials are provided in the Sync section of the MIDI output port.