You will find mines located around the realm. Mines contain Minerals, Ore and Gems which you can extract with your armies.
 
Each mine has a central structure surrounded by nodes. Each node has a limited amount of mines resources.
 
You can tap on any empty node and summon an army to start extracting resources.
 
Once the army is full or the node is empty you can recall them home to deposit the resources they extracted.
 
If a mining army is attacked and defeated by another player then the attacker loots some of the resources the army extracted and the mining army returns home with whatever they have left.
 
Once a node is fully mined it collapses and a new one appears in its place.