Transport commands can be customized with Preferences > User Interface > Edit Keyboard Shortcuts and configured with Playback > Audio/MIDI Setup > MIDI > Remote Transport Control.

Transport Bar

Rewind: Reset playhead to the very beginning.
Stop: Stop all playback.
Pause: Halt playback at current position. Click again to continue playing from there.
Play: Start or restart playback (Space). If something is selected on the Time Ruler or in a Parameter View, playback starts from there. Hold down Alt to play only instruments with phrases in the current container.
Solo: Start playback, rendering only the currently selected tracks. This is different from soloing the audio signal with the S button.
Preview: Listen to the currently selected object by rendering an example phrase. You can also use this to preview Harmony or any other parameter that's currently selected. Enable Playback > Tie Auto-Chord Notes to sustain all notes that are already playing in the previous chord.
Record: Record the currently selected phrase from external MIDI input. See: Recording.
Record Parameter: Record anything from external MIDI input and convert it to the currently selected Parameter. See: Recording Parameters.
Loop: Run next playback in cycles as marked on the Time Ruler.
External Sync: Transmit TEMPO, LOCATE, START, STOP commands to external hardware in synchronization with local playback. See: External Synchronization.
Overdub: Don't clear the phrase before recording, i.e. add whatever is recorded to the existing phrase.
Bounce to Disk: Save audio output of next playback to a file. The file is placed next to the arrangement document (File > Show File in Finder). Requires all instruments to use the same Audio Engine.