Xanthan gum is used to replace the gluten in gluten free recipes. It acst as a binding agent so whatever you bake does not crumble into pieces. Too much makes a baked product slimy. If a recipe does not specify how much xanthan gum to add to a flour mix or if you are winging it and making up your own recipe, you can follow this guide - for each cup of gluten free flour mix you use in a recipe, add:
1 to 1 1/2 tsp. for cakes and quick breads
2 tsp. for breads or pizza crusts
none or 1/2 to 1 tsp. for cookies
Guar gum can be used in place of xanthan gum. Use the same amount of guar gum as you would use xanthan gum.