Resource production is the most basic but also key element to building a strong Kingdom. It is vital to remember that there are Heroes with skills that increase resource production passively once they are upgraded.
Heroes with Yield Skills
Food Yields Increase
Gold Yields Increase
Metal Yields Increase
Some skills require a Hero to reach a certain level before they can be used. If the skill you want to use is locked, you will need to level up your Hero and unlock their power.
Visit Kingdom Hunter's Official Guide for more information about the game.