The bowyer skill is used to make wooden and bone crossbows at a bowyer's workshop. A dwarf whose highest skill is crossbow-making has the profession of bowyer. A higher level of skill produces higher average quality crossbows.
Although crossbows can also be created from metal by a weaponsmith in a forge, the damage a crossbow deals when fired is determined by the weapon's quality and not their material. The material only affects damage when dwarves swing their crossbows as impromptu hammers during melee combat.
This skill is not used to make bolts, which requires either wood crafting for wooden bolts, bone carving for bone bolts, or weaponsmithing for metal bolts. It is also not used to make metal crossbows at a metalsmith's forge, which again requires weaponsmithing.
While they can normally only craft crossbows, a bowyer who goes through a strange mood has a chance to create an artifact blowgun instead; this is the only way to acquire a blowgun outside of looting dead cavern-dwelling animal people. Note that strange mood artifacts do not come with ammo included, and therefore these blowguns will not be actually usable in combat.