A despot, is a cruel, all-controlling ruler. For example, a despot does not allow people to speak out against the leadership, nor really want them to have much freedom at all.
The word despot came into English in the sixteenth century from Old French, but it traces all the way back to the Greek word despotes, meaning "master of a household, lord, absolute ruler." The word is often used to describe someone who abuses power and oppresses others. Obviously, it's not a nice thing to call someone, especially within earshot of the despot who has absolute power over you.