Can someone please explain the combat runes

[Loopy]

It's a 1.1/1.2/1.3 multiplier to the effect of the rune. i.e. Attack runes give +22/24/26% damage and crit rate.

It's worth noting that it's very easy to grind this medal every battle. I had the 2nd level before getting off the first island.

It's also worth noting that while the benefit only applies to the base stats of the units, other buffs can work on both. My Berserkers with +1 morale have an 82% crit rate from both Berserk and Attack rune.