Gluten is a protein in flour that helps the dough form elastic threads, which contain air bubbles as the dough rises. That is why when baking gluten free yeast bread the result is often crumbly and hard.

Psyllium Seed Husk performs much the same function as gluten and is therefore a useful replacement when baking. The resulting bread will be moist and light in texture.

It is just not possible to bake gluten free without it!

I use the Psyllium Husk Powder 250 g from Whole Foods (can be purchased via Amazon or directly from Whole Foods) but there are several other suppliers.