Forging Potential is a property that all normal, magic, rare and exalted equipment items have, which is used at the forge to perform crafting actions. Once an item runs out of Forging Potential, it can no longer be crafted on but is still able to be used. Higher rarity items will generally drop with more Forging Potential.
Each time you use a shard or rune in the forge, the Forging Potential of the item affected is reduced by a random amount inside a range. The amount of Forging Potential used can be modified through the use of Glyphs. As long as you have any amount of Forging Potential remaining, you are able to craft on the item in the forge.