Baileys ice cream stabilizer
I've used xanthan gum with great success. You can get it in powdered form at places like Whole Foods and health food stores. You might find guar gum also, although I haven't tried it. The teaspoon measurement is about right for a standard home recipe that makes a quart of ice cream, although you can add a quarter teaspoon at a time and stop when the consistency looks about right.  Add it while spinning the mix in a blender to prevent clumps. If you add too much, it'll become thick and sticky like peanut butter. 

I'm assuming they're calling for some kind of stabilizer because Baileys has a lot of water in it, but you may find that you don't need the full teaspoon.