xanthan gum

I have been wheat and gluten free for a year. With cakes and biscuits I have been using gluten free flour and my regular recipes as the gluten free cook book I have has pretty boring recipes in it. (cakes and biscuits). Firstly, I don't know what xanthan gum is or why I need to use it. Some of the recipes have worked well, others not so well. Can you provide me with a rule of thumb which might work in converting an ordinary recipe into a gluten free one. I have so far added extra egg and this has worked okay with some recipes, but turned another biscuit into a slice and the chocolate cake I  made didn't freeze well and I had to throw it out upon thawing. I don't like to be fussy, but my recipes are a part of me and I'd like to adapt them. I am perfectly happy to use gluten free recipes for new things. I hope for some help. Thank you Penlap.