Differential Score shows you the difference between your score and the course's Par if you're playing in Stroke Play. It also shows your points if you're playing in Stableford.
When starting a round in 'Scoring' mode, you can decide to hide or to see this information. On the 'Hole Map' screen tap the 'Settings' icon on the top right. There you can turn the 'Differential Score' toggle On or Off.
The Differential Score will be displayed at the top of the scoring screen.
Likewise, on your Apple Watch, it will also appear on the scoring screen.