Supreme Court v. Parliament: The Basic Structure | लॉ वार्ता #3

On 24th April 1973, a 13-judge bench of the Supreme Court of India laid down the "basic structure" doctrine.
What is the doctrine all about? What did the Supreme Court hold about Parliament's powers to amend the Constitution? What can the Parliament do, and what can it not? Why is this hotly contested, and why does all this matter?