The phrase "to flavor the syrup" means to add a specific taste or aroma to a liquid sweetener by mixing in ingredients such as fruits, herbs, or spices. Full definition
Some things that worked was warming it up, using milk or yogurt to make a fruit smoothie, or adding a small splash of flavored syrup to make it more tasty. (babycenter.com)
I love them with sugar free maple flavored syrup and strawberries. (ibreatheimhungry.com)
If you want to flavor pancakes, cake, bread, etc. use sugar - free flavored syrup instead. (margeburkell.com)