Forge Bonus is either a multiplicative or an additive discount on Magic Items.

Here are some examples:

• The Titan of the Forge has Forge Bonus (20%), so he pays 20% fewer Gems & Blood Slaves to make Items.
• The Master Smith has Forge Bonus (2), so he pays 2 fewer Gems & Blood Slaves to make Items.

This is not to be confused with the Master Smith trait, which is a totally-different skill.