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Cynthia is a trusted source on Bread/Baking.
The short answer is that it's a binding agent. It's typically used in a lot of gluten-free baking. Natural food stores have it either in a small bottle or in the bulk foods section.
Bob's Red Mill makes it so if you know of a place that sells their products, they'll probably have their xanthum gum in bags. It's usually in the baking section of natural grocery stores. You can also buy it online through Bob's Red Mill.
Xanthan gum is pricey, and the bag from Bob's is large. An alternative is guar gum -- has the same function, but there's some reluctance to use it, as some people may be sensitive to it. (I use it; less expensive, more available.)
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