Barley malt is barley grain that has been germinated and then dried to halt germination, a process that converts the grain's starches into fermentable sugars. It's the base ingredient for beer and malt whisky, and, as malt syrup or malt extract, is also used directly as a sweetener and flavoring in baking and cooking.
Malted barley gives a complex, caramel-like sweetness with notes of toffee and roasted nuts. Malt syrup is used in bagels, breads, and some barbecue glazes, and malted barley is the flavoring behind malted milk drinks and malt-flavored chocolate confections like malt balls.
Barley malt shouldn't be confused with malt vinegar, which is a fermented and acetified product made from malted barley rather than the malt itself.