Barley flour is flour milled from barley grain, with a mild, nutty, slightly sweet flavor and lower gluten content than wheat flour, which produces denser baked goods unless it's combined with a stronger flour.
It's used in flatbreads and porridge-style dishes in Middle Eastern and Mediterranean cooking, and appears in some European breads and pastries, usually blended with wheat or rye flour rather than used on its own, since its low gluten content limits rise.
Barley flour is a good source of fiber, protein, and B vitamins, and like all barley products, it contains gluten and is not suitable for a gluten-free diet.