Gluten is a protein and is found in grains such as spelt, wheat and rye. Gluten is a useful when making bread and pasta as when the flour (which contains gluten) is mixed with water, it creates a glue-like “sticky” consistency, making the dough strong and elastic. It gives bread a delicious chewy texture. Salt also helps to preserve the strength of gluten.
On the other hand, when butter is added to a dough mixture, it shortens the strands of gluten, making the dough “short”. Butter contains fat, and the fat weakens the strands of gluten, so they can’t join together to make a strong elastic dough. This is why butter is added to dough such as brioche, short pastry or baking which require a soft tender mixture.