I call it enchanting for a lack of a better term. In this process, which is similar to Upgrading, you give a weapon an element. This is useful because you can use the system of elements to deal much more damage to the monsters you are fighting at the time.
For instance, if you give your sword a Water Element +1 and go fight Creep Nyangnyangs (Which are fire creatures) you will deal a significant increase in damage. However, if you fight Nyangnyangs with this weapon, you'll deal much less damage, as they are electric monsters.
To Enchant a weapon, double click on a level 2 card and then click on the weapon. If the process succeeds, your weapon will say in its description [Whatever Element] + 1. Once your weapon element reaches +5, you will need to use level 3 cards to upgrade further. Do note however, that dice and card levels do not conflict with each other. So, for example, if your weapon is a regular +7, but your element level is a +2 Water, you still need to use a level 2 card to upgrade, because only the water is below +5. You'll still have to use a Dice 6 to upgrade the regular + to +8.
Enchanting your weapons to +4 gives them a special glow according to element. This glow increases as the enchant level goes higher.
If the process fails, you WILL lose your card and your weapon unless you use Sprotects which protect them from breaking.