Mana is earned when you pray to a Statue. To pray, left click a miner and then right click on your Statue. You will earn 7 mana every 2 seconds. Mana is used to cast spells, use abilities, and train troops.
Mana is the magic of your empire. This is how magic in your empire is used.
Mana helps you use your new technologies.
If your miners pray you get Mana.