Differential Score shows you the difference between your score and the course's Par.
When playing in SCORING mode, you can decide whether you want to see or hide this information. On the screen of the hole map, after starting the round, tap the bottom bar to open Settings. There you can turn the 'Differential Score' toggle On or Off (as illustrated in the middle image below).
Your Differential Score will be displayed at the top of the Scoring screen (right image below).