Each civilization has their own runic symbol and power. This symbol you see here is the runic symbol for the Greek civilization. The runic power won’t affect the first set as much as it will later sets. In order to play a card from that civilization, you must have the matching runic power to do so. For example, let’s say the next set is Aztec. The Aztec will have their own runic symbol and if you decide to build a deck with some Greek and some Aztec cards, you will then have to play the correct runic power during your turn in order to play those cards into the battlefield. This will be explained in great detail when the second set gets released, as it is not a factor for the first set since all you can choose is Greek cards currently. Each civilization’s runic symbol doubles as a set symbol too.