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.