Limoncello is an Italian lemon liqueur taken as an after dinner drink which is referred to as a "digestivo". This popular liqueur is produce in Italy, most famously in Sorrento and Capri but is produced in other areas as well. The typical alcohol content is about 28% to 32%.
The drink is rather thick and very sweet. For drinking the liqueur should be stored in the freezer so it is icy cold. In addition to be used as a beverage it is also used in many desserts including sorbets, cakes and cookies. It can be used as a simple topping for ice cream. It's used occasionally in savory dishes.
If you don't have Limoncello you have a couple of options but if is a main ingredient in the recipe you may want to wait until you can find it: