This command is only enabled if you have indeed selected part or the entire active signal in the graph window. When activated this command plays the selected portion of the signal.
This command is also available from the play button on the toolbar at the bottom of the graph pane: if a selection is active, that selection will play, if no selection exists the whole file will play when pressing the play button.
play button on the toolbar at the bottom of the graph pane: if a selection is active, that selection will play, if no selection exists the whole file will play when pressing the play button.