It is an unfortunate fact that in the heat of battle troops often don't respond as you, their commander, might want them to. Faced with terrifying supernatural foes, their courage might fail, or they could simply be too dim to understand the orders they have been given. As the army commander, it is your duty to know about these things and take them into account. If you do not, then you may find that your best plans may not go exactly as you were expecting.
The Psychology rules are a set of special rules that represent these factors in the game. Some Psychology rules call upon the player to make occasional tests to determine whether his troops are affected by adverse psychology, such as Fear or Terror. Other Psychology rules, such as Frenzy and Hatred, always apply and do not require a Psychology test.