Malted Barley Flour is made from barley that is malted (e.g. barley that is allowed to germinate), steam-dried, hulled, then ground and sifted.
Technically, because it's malted, it has increased enzyme activity. More precisely, it contains alpha amylase, which helps to break down complex sugars and starch in the dough to simple sugars such as maltose, making them easier for yeast to feed on.
It is used commercially a great deal in bread, pizza crusts, crackers, rolls, pretzels etc. as a dough conditioner -- because of its lower gluten, it causes the dough to be softer, more relaxed and gives a softer crumb texture.
Malted Barley Flour contains a small amount of gluten, and a good bit of fiber.
It contains some tannins -- some people who are very prone and sensitive to migraines may find that the tannins aggravate their headaches.