The Supreme Court acts as the Custodian of Constitution of India because the Constitution itself has empowered the Judiciary to review the laws enacted by the Parliament. Article 14 of the Constitution states that if the law enacted by the legislature is ultra vires to the Constitutional provisions, the Supreme Court gets the power to review and repeal it.
The Supreme Court also acts as the guardian of fundamental rights by declaring any law that infringes upon these rights null and void. If the Supreme Court observes that the fundamental rights as given in the Constitution are being violated, it is empowered by Article 32 of the Constitution to issue writs to enforce them. These writs are in the nature of Habeas Corpus, Mandamus, Quo-Warranto, Prohibition, and Certiorari.