Gluten is the combination of proteins in certain grains (wheat, rye, barley...). In the culinary world, it acts a “glue” to keep mixtures together and helps maintain shape.
Why does it bother people?
Gluten is a natural protein in grains, so it isn’t bad. However, over the years, we have started to process foods more and more in order to increase shelf life and artificially breed crops to be more consistent. Gluten, after being highly processed, becomes a red flag for many people’s bodies, signaling an autoimmune response effecting the GI tract. Grains with gluten have been used for thousands of years with no problems, but because of how processed modern foods are, many of our bodies have started rejecting it.