It's a frothed topping Starbucks adds to some of their drinks. They take sweet cream (the creamy liquid used in their sweet cream cold brews), froth it up, & top drinks with it.
Put water & sugar in a saucepan & bring to a boil. Lower heat & let simmer for about 10 mins until sugar is completely dissolved & mixture has thickened a bit.
Place syrup in fridge & let steep for at least 6 hours. Syrup will thicken as it cools. If you want a more intense vanilla flavor, you can leave the bean in.
Turn mixture into cold foam right before using with either use an electric frother, hand mixer, or blender. Add sweet cream & froth, mix, or blend until mixture turns into a foam.