Hi,
a rune is simply a spell, but they interact with other creatures in special ways. There are four different runes. As a spell, they each cost 2 mana and can either:

1. heal one of your creature and your god for 6
2. deal 3 damage
3. give +2/+2 and frontline to a creature
4. foresee two cards and draw a card

There are a lot of cards that give you runes (like yours). Other cards profit, if you have some runes in your hand, e.g. Ocular Fiend gets +1/+1 for every rune in your hand :)

Hope that helps...if you want to know some more, feel free to ask here, contact me on discord or check out my blog...there are also some GU riddles there that help you understand the game better ;)