For that matter, what’s gluten? The next time you make a batch of bread or pizza dough, pinch off a little bit and work it between your fingers under the kitchen faucet for a minute. A good proportion of the dough, mostly water-soluble starch, will wash away. Yet a small rubbery ball will remain. That’s it. The non-water-soluble, protein portion of flour: gluten.
Of course if you tried the same thing with just a pile of flour or a simple water-flour paste the whole thing would wash away. That’s because gluten must be both watered and worked in order for it to organize itself into a mass.
What we call gluten is actually a combination of two different proteins: glutenin and gliadin. Both are extremely long-chain proteins, but with different properties. Glutenin molecules are rather fluid, and are capable of forming very strong bonds with one another. When they’re worked they do just that, bonding both end-to-end and side-to-side into a kind of mesh or network. Gliadin molecules by comparison are tightly wound and bond weakly to one another and to glutenin molecules.
The elastic mesh that these molecules form is what allows dough to rise. The gluten mesh catches and holds carbon dioxide bubbles made by yeast, which would otherwise simply evaporate. The gas bubbles thus make small pockets in the dough. As the dough heats in the oven, those pockets begin to expand, partly as a result of heating gas, but mostly as a result of steam. The stretchy gluten mesh expands with the gas and steam until the starch in the dough gelatinizes, fixing the bubbles in place.
Just how big the bubbles get is determined by two things: the protein (gluten) content of the flour and the amount of water in the dough. More gluten provides more elasticity, allowing bubbles to expand, and more water makes a softer dough, allowing those bubbles to more easily combine with one another.
Of course the elasticity of gluten also makes breads chewy. Sometimes this is desirable (bagels and pizza crusts), sometimes it isn’t (cakes and biscuits). Thus we have flours with different proportions of protein for different purposes.
So-called high-gluten flour has the highest protein content of any standard wheat flour (only semolina flour has more), and so has very few uses for most home bakers. So few, in fact, that it’s not worth the time for most commercial flour makers to package and sell it in grocery stores. You either need to order it, or somehow con your local bakery or pizza parlor out of some.