Relics are mystical stone with power in it. They range from increasing your power, speed, critical chance, agility, armor, giving you karma, giving you more exp from battles, giving you more gold from battles. You need relic slots which can be purchsed by karma to equip more. Currently, there are 6 slots available. You start with 1. Then you buy more for 2, 5, 10, 20, and 50 karma.
Relics are usually quite expensive (for example, a level 28 weapon may cost about 1.2 mil, while, the level 28 relic, Green Kunai Star is 6 mil.)