Why are students protesting across India? Why is NRC a big controversial topic?

India has been seeing massive protests throughout this week and the media claims that this will grow beyond our imagination. The situation is rapidly growing to a violent situation probably running out of the control of the govt. The protest was started by the students of Jamia Milia University, Aligarh Muslim University, and the Delhi University. But understanding why this has been happening for a long time is certainly a big question mark.