It is used in various mixed alcoholic cocktails.
You can buy it in bottles, or make it yourself.
To make: Bring to a boil 1/2 cup (4 oz / 125ml) water with 1 cup (8 oz / 225g) white sugar, and let it gently boil for about 5 minutes. Remove from heat, then stir in 1/2 teaspoon of almond extract, and 4 tablespoons of butter until the butter has melted.
Let cool, store in the fridge in a sealed container.
Orgeat; another nut-flavoured syrup.