To dethrone means to remove a king or queen from power, like when Mary, Queen of Scots was kicked out of Scotland. You can also dethrone someone less formally, as when you dethrone your school's fastest runner by beating her mile time.
In today's politics, to force a leader out of office is to depose them — except in the case of a monarchy, when we use the word dethrone. It literally means "to remove from the throne" and therefore refers specifically to those who sit on thrones: namely, kings and queens. You can also use this word in a figurative way to mean "remove from a dominant position." So, for example, you can say that the new jump rope champion dethrones the previous record holder.