President's rule refers to Article 356 of the constitution of India and is invoked if there has been failure of the constitutional machinery in any state of India.
In the event that government in a state is not able to function as per the constitution, the state comes under the direct control of the central government.
Article 356 gave wide powers to the central government to assert its authority over a state if civil unrest occurred and the state government did not have the means to end the unrest.