Overview
Bolt has a few of meanings:
- In World of Warcraft it almost always means a roll of cloth processed by a tailor.
- Sometimes it can refer to an item associated with lightning, as in a bolt of lightning.
- In other games, a crossbow (ranged weapon) might use ammo with this name. In World of Warcraft, crossbows use arrows.
Bolts of Cloth
A Bolt of Cloth is used by tailors. They get various amounts of normal cloth and can convert it into a bolt, used in almost all tailoring recipes. 
[Felcloth] is a special cloth, however - two of them every four days at a moonwell can be turned into 
[Mooncloth] as opposed to a Bolt of Felcloth (non-existant).