When you dictate something, you are giving orders — acting a bit like a dictator.
Dictate can also be used in a broader sense. Weather often dictates how many layers of clothes you wear, and the school rules dictate how you behave in and out of class. In a very different meaning, dictate means to say something out loud to a person or into a machine so it can be recorded. Up until fairly recently, bosses dictated letters and reports to secretaries who wrote them down.