Resources are the building blocks of the economy (I don't know if that's true it just sounds cool). Anyway, civs will try to get resources, and then they will use these resources to become more powerful. For now it'll be fairly abstract to keep things simple. For example all mountains will have a certain amount of resources in them.

Metals

Found on mountain tiles. Used for industry, including weapons and armour. Dwarves are especially effective at mining it, and can trade it for profit.

Food

Used to sustain populations, either through hunting/gathering or agriculture. Civs can build farms on some tiles to increase output.