By enabling looping, playback does not stop when it reaches the end of the project. Instead, it restarts from the predefined loop start point once it hits the predefined loop end point. Since looping is a concept related solely to the Timeline, it is not available if there are no clips placed on it; as on the screenshot, looping button is disabled.
Form A Loop
When the mouse is over the Timeline track lines, hold Ctrl/Cmd and drag while holding down the left button to enable looping. Alternatively, click/tap the looping button on the header. The default loop start is at time 0, and the default loop end is set to the project’s end time. On touch devices, a left click will always open the properties dialog; on non-touch devices, the dialog opens with a right click.
Loop By Markers
Timeline markers can be selected as a reference for loop start-end time. At the moment, moving the markers does not impact the already selected period.
Dissolve Loop
Either click/tap the looping button in the header or Ctrl/Cmd + left-click on the track lines. The start and end point values remain the same, so re-clicking the button applies the same values on non-touch devices.
Note: loop settings are not stored in the project, or as a user preference; when you reload / close the desktop app, selection dissolves by itself and start-end point values are reset to default.