Enhancement Rune
Enhancement Runes are items that are used on Legendary Item Traceries in order to increase their Item Level and improve their stats or buffs. They were introduced in update 30.3 as part of the new Legendary Item system. Enhancement Runes function similar to how Heritage Runes functioned in the old Legendary Item system. Note that only stat bonuses on Traceries are improved with Enhancement Runes: skill buffs on Word of Power and Word of Mastery Traceries are not improved with Enhancement Runes and are based on the quality of the Tracery instead.
Enhancement Runes are available in different qualities (or rarities):
Uncommon,
Rare,
Incomparable,
and Epic.
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For a complete list of Enhancement Runes see the Enhancement Rune Index.
Obtaining Enhancement Runes
Enhancement Runes can be acquired as quest rewards, drops from bosses in instances, or bartered for at Gobeth Teithian, the Archive of Traceries in the Last Homely House in Rivendell.
Applying Enhancement Runes
To apply an Enhancement Rune to a Tracery, first open the Legendary Item Panel by dragging the Legendary Item to the 
Socketed Equipment slot on the Character Panel or by Shift + Double-clicking the Item. Then drag an Enhancement Rune (or stack of Runes) onto the Tracery's slot in the panel. Each Enhancement Rune raises the Tracery's Item Level by one. Applying a stack of Enhancement Runes to the Tracery will display a prompt for how many Runes in the stack to apply (up to the Enhancement Limit described in the next section).
Enhancement Limits
Tracery Item Levels are capped at the Item Level of the Legendary Item in which they're slotted. Reforging a Legendary Item will increase its Item Level.
Enhancement Runes are limited in how high they can raise a Tracery's Item Level based on their quality and level (indicated by the "Enhancement Limit" on the Rune's tooltip). Uncommon quality have a lower Enhancement Limit than Rare quality Enhancement Runes, which similarly have a lower Enhancement Limit than Incomparable quality Enhancement Runes of the same level. Therefore, when applying Enhancement Runes to a Tracery, always start with the lowest quality Runes first.
Example: A level 130 character has a Legendary Item that is Item Level 449 and all of the Traceries on it have been enhanced to their maximum (Item Level 449, same as the Legendary Item). That character then levels to 131 and they reforge their Legendary Item, making it Item level 465. Now the Traceries on the Item can be enhanced to Item Level 465, so the character applies a stack of Uncommon-quality
Enhancement Runes, Lvl 131. Only eleven of the Runes are applied from the stack though, raising it to Item Level 460 (since the Enhancement Limit for the Uncommon-quality Rune is 460). In order to max out the Tracery the character must then apply five Rare-quality
Enhancement Runes, Lvl 131 or better to get it to Item Level 465.
Item Level increases applied to a Tracery slot remain on the slot even if the Tracery is unslotted. If another Tracery is then slotted in that same slot, it will inherit the upgraded Item Level. The Item Level on the slot is visible in the Legendary Item Panel to the right of the slot while a Tracery is slotted (it will say "L-#/#", with the numbers indicating the current and maximum possible Item Level of the slot).
See the Enhancement Rune Index for a list of all Enhancement Runes along with their Enhancement Limits.