The concept of judicial activism is closely related to the concept of Public Interest Litigation (PIL). It is the judicial activism of the Supreme Court which is the major factor for the rise of PIL.
In other words, PIL is an outcome of judicial activism. In fact, PIL is the most popular form (or manifestation) of judicial activism.
The Public Interest Litigation (PIL) is a strategic arm of the legal aid movement which is intended to bring justice within the reach of the poor masses. It is a device to provide justice to those who individually are not in a position to have access to the courts.
To read more about the Public Interest Litigation (PIL) in India, aspirants can check the linked article.