When equipped in the other weapon slot, Bolts allow you to use Crossbows.
Key details
They are an ammunition item type and come in stacks of (up to) 500. may be crafted using Recipe: Bolts or purchased from NPCs.
D2Emu Tips
Crossbows cannot be fired if Bolts are not equipped.
Bolts are required to use Attack or other attack-based skills while equipped with a Crossbow-class weapon, unless the user casts Magic Arrow or is in Werewolf or Werebear form.
Classic D2 manual: "When using ranged weapons, an icon of “crossed Arrows” appears on the Play Area whenever your quiver is running low on ammunition.".