Gluten is what holds are your breads together. It is a protein in grains like wheat, barley, rye, semolina, and others (and oats by cross-contamination). A lot of people have an intolerance of some sort to it. I think it's somewhere like 1/125 people. Gluten-free flour is made from grains or other things that don't have that protein, like rice, millet, quinoa, buckwheat, soy, garbanzo, fava beans, etc. The problem with gluten-free flours is that they don't hold together very well.