Every weapon or piece of armor has a certain number of CP. When you enhance it with a smithing ability, its CP decreases. As a result, smithing abilities can only be performed a limited amount of times on any one piece of equipment.
Click on either the "Smith Weapon" or the "Smith Armor" link to choose the item you want to smith on.
The effect of a smithing ability will be listed in the Lucent Ability Research List.