A byoo is special type of tack used to fasten the head onto a Taiko drum. It lends its name to any type of Taiko drum which has a tacked on head - Byoo-daiko as opposed to Shime-daiko which attach and tune their heads with rope.

If you need a mental image of what a tacked on drumhead looks like, think of a tambourine--it too has a tacked on head.