Relics are items and equipment that you gather along your path. There are certain relic requirements per minion and per rarity that you need in order to upgrade your minion.
Relics themselves also have different rarity levels to look for. The more common relics can be found during specific battles. Some of the more rare relics will require you to craft them out of other relics. The most legendary of relics can be found during live and special events.