Xanthan gum is one of the common ingredients you will see in gluten free recipes. It acts like our gluten, adding bounce and binding to breads and cookies that would otherwise crumble. Xanthan gum is a powder milled from the dried cell coat of a micro-organism called Xanthonomonas campestris, grown under laboratory conditions. In the US Xanthan gum is grown off of corn syrup (but tests out corn-free in the lab after processing).
Most gluten-free flours will require the addition of xanthan or guar gum (another gum binder) to compensate for the lack of gluten. The amount needed to add will depend on the type of product you are baking and it’s reliance on the gluten structure. Breads rely heavily on gluten for their structure, cakes to a lesser extent, and cookies almost none. Typically the starchier and/or more refined the crumb, the less the reliance on gluten.
Some gluten-free groups discourage the use of guar because of the higher fiber (and therefore possible laxative effect) of large amounts of guar gum use. The best way to know which gum to use for your binder is to try both and see which you prefer. I generally use xanthan as it is much easier to find (and afford!) in my area. If these gums are not appropriate for you, some suggest the use of mung bean (also known as green bean) flour (as a rate of 1/8 of cup to every cup flour) or pre-gelled potato flour. I have also seen some experimentation with chia seeds, but have not tried that myself.
The suggested ratio for every cup of wheat-free/gluten-free flour use:
1 tsp. to 2 tsp. Xanthan gum for pizza
3/4 tsp. to 1 tsp. Xanthan gum for breads
1/2 tsp. Xanthan gum for cakes
1/4 tsp to ½ tsp. xanthan gum for most cookies.