Vital wheat gluten is a dried wheat-protein product that develops an elastic network when hydrated. Bakers may use it to strengthen certain doughs, and it is a main ingredient in many seitan recipes.
It is not ordinary bread flour, which contains much more starch, and it is not a gluten-free ingredient. It is a particular commercial form of wheat gluten, rather than a synonym for the finished cooked meat substitute.
Reference: Further reading.