Martial law is the imposition of military power on a state on an emergency basis.
When the civil authorities or officers fail to function correctly, the highest officer in military can over throw the civil authority, take over the responsibility of the state, as the military governor or head of the government.
During the martial law, the usual civil law and civil rights are suspended. Any protests and protestors are dealth seriously and punishments are very bad.
pakistan had military rule or martial law some years ago.