This Ableton tutorial covers the Back to Arrangement button and how it controls playback priority between Session and Arrangement views. Once you've recorded clips and scenes into the Arrangement view, clicking the button tells Live to play the timeline rather than any active Session clips.
If you trigger a clip or scene from the Session view during Arrangement playback, Live switches to following that instead, and the Back to Arrangement button reappears highlighted in orange as a signal that the timeline has been overridden. Clicking it again hands control back to the Arrangement. For cases where only one track was triggered from the Session view, you can return just that track to Arrangement control without affecting the others.