Trimming marks. There are put on sails with free flying luffs like screechers and assymetrical spinnakers. Supposed to help you know if the luff is folding particularly at night.
Waste of time in my opinion. If you can't tell the luff is folding without them you've got issues with your eyesight or you hearing. People have seen them on the volvo boats so they want them.
It probably works better on a sail that is less transparent!