Different sets of equipment have been very important in both battlefield and resource gathering. The feature Equipment Refining allows you to strengthen each part of the equipment separately in your desired way. You can refine equipment at your Blacksmith, on the Refine tab.
Each equipment part offers 4 slots, representing Green, Blue, Purple and Golden quality (Uncommon, Rare, Epic, Legendary). A slot of a certain quality will be unlocked if you have the same level of that type of equipment and a random bonus will be granted to this slot. You can choose to strengthen this bonus with Refinement Ore (the essential item that can be acquired through Expedition Beyond), or change it with a reset item.
To raise the cap of the bonus, you must unlock certain research in Equipment Refining in Maester's Tower. Don’t forget to research!