This is a polysaccharide secreted by a bactreria (Xanthomonas campestris). It is used as a thickener and only small amounts are needed. It is poisonous to dogs, so always check ingredients labels before sharing with your four legged friend.
Xanthan gum is also used in gluten free baking to give the dough or batter a stickiness that can otherwise only be achieved with gluten.