Gluten
Gluten is the protein network that forms when glutenin and gliadin, both present in wheat flour, combine with water. Glutenin provides elasticity, gliadin provides extensibility; together they build the structure that traps the gas produced by yeast and bacteria.
The network develops two ways: mechanically, through mixing and folding, and on its own, as the dough rests and the proteins link up without intervention. Autolyse is built on exactly that, leaving flour and water alone before salt and starter go in.
How much gluten a flour offers, and of what quality, varies. Strong bread flour has more protein and holds higher hydration; rye forms almost no gluten network at all, which is why rye dough behaves differently and relies on pentosans instead.