There are three currencies in Thor: War of Tapnarok. The most abundant by far is gold coins. These pour out of your enemies like shiny yellow guts as you hack them to death, and you spend them on unlocking allies, unlocking skills, and levelling up everything.
Then there are runes. You can spend runes on permanent buffs such as Idun’s Bronze Apple, which increases damage and gold collection, and the Yellow Rattle, which increases damage and weak point damage.
Once you’ve unlocked one of these rune buffs you can level it up with more runes, and you can break down buffs to reinvest the runes in something else.
Finally, there are gems. These let you buy one-off powers and perks such as Mimir’s Well, which costs 124 gems and gives you 26 runes, Protection of Svalinn, which costs 50 gems and protects your allies for 24 hours, and Helm of Awe, which costs 75 gems and removes a creature per stage until the next time you Time Travel.
Gems and runes come along in small numbers through combat, but to really net a lot of runes you need to travel back in time. More about that later.