Xanthan is a polysaccharide, which is a type of molecule made up of sugar molecules. It is commonly used as a thickening and stabilizing agent in various food and cosmetic products. Full definition
Hey Jamie, I believe a scant 1/4 teaspoon of xanthan gum added to this recipe would be a good place to start. (yupitsvegan.com)
For most recipes, you'll also want to add 1/2 to 1 teaspoon xanthan gum for each cup of flour blend used. (glutenfreeandmore.com)
The photo on the right (taken by the fantastic amazing Shauna over at Gluten Free Girl) is an example of what happens when you don't use xanthan gum in a cake. (eatthelove.com)