Weapons in MA:O can be enhanced by visiting your guild's Master Crafter.
Enhancements are special modifiers that can be applied to weapons. This can be something like adding extra blunt resistance, or faster attack speed, or even bonus stats like affinity or luck. They are applied by visiting a Master Crafter, or by using an Irisite Anvil.
Enhancements typically encompass a range of possible values, i.e “Blunt Resistance +2 - +8”. This is because when enhancements are applied, their effects are then randomly given from the pool of possible stats as dictated by the incoming enhancements.
Enhancements can be removed, for a cost - but you must remove all enhancements from a weapon, and can not selectively remove enhancments that have already been applied.
Applying enhancements costs scraps, which are currency earned from deconstructing any piece of equipment in a furnace. Removing enhancements will cost Gold Coins.
You must learn an enhancement before you can begin applying them to weapons. Furthermore, each enhancement can only be applied to specific weapon types, and only at certain weapon levels. I.E, you may not be able to apply a Martial Lvl3+ enhancement to a Martial Lvl1 weapon.
There are a few ways to unlock enhancements.
Crafting and Deconstructing weapons is the primary way to learn new enhancements. Again, once an enhancement is learned once, you can then use it whenever you like when enhancing weapons.
To check if a weapon has an enhancement you can learn from it, either check a craft by hovering over its craft icon, or hover over a weapon while in the furnace's deconstruction interface. Doing so will reveal whether or not there is an enhancement to learn – you will know there is if you see a “LEARN: X” window appear next to the item's normal description window.
You can check on enhancements you have unlocked, as well as those you have yet to unlock, by navigating to the “Enhancements” panel of the character menu. Doing so will show you a list of all possible Enhancements in the game. Clicking on an enhancement in this panel will show you which weapons can be deconstructed/crafted to learn the given enhancement, and hovering over the items will show you the percent chance you have at learning the enhancement.
Scraps are the currency used to apply enhancements to a weapon. You will need a lot of scraps if you want to get the optimal enhancements applied to some weapon. They can be obtained by deconstructing any piece of equipment - armor or weapon.
Once you have some enhancements unlocked, you can apply enhancements by visiting your guild's Master Crafter, or by finding an Irisite Anvil. Then, you can add the enhancements you want, and click “apply” when ready. You can get a detailed breakdown of the possible stat ranges your enhancement will apply by clicking the “Show Breakdown” button in the enhancement interface.