Fortify One-Handed is an effect in The Elder Scrolls V: Skyrim that increases the damage caused by most one-handed weapons. Fortify One Handed enchantments and potions do not affect daggers unless you have the Unofficial Patch installed(however Fortify Marksman Potions affect all weapons including daggers unless you have the Unofficial Patch installed).
Fortify One-Handed can be enchanted into apparel that goes on your neck, hand, finger and feet if the enchantment has been learned. The effect increases your One-Handed damage output by the percentage listed on an item. Multiple items stack(4 items that increase One Handed damage by 40% each will increase the damage output of your One Handed weapons by 160% total).
Potions with Fortify One-Handed can be brewed by combining certain ingredients at an Alchemy Lab. for 60 seconds they will increase your damage output with One-Handed weapons by the percentage displayed in your inventory.