Orgeat syrup is a sweet syrup made from almonds, sugar and rose water/orange-flower water. It was, however, originally made with a barley-almond blend. It has a pronounced almond taste and is used to flavor many cocktails.
Orgeat syrup can be hard to find, but can be found in some stores that sell coffee syrups/flavorings. If you cannot find it, almond syrup is a good substitute for orgeat syrup.