What does it mean for a court to be activist?

What constitutes an appropriate role for the judiciary? Some people argue that courts have become too powerful and that judges legislate from the bench. What does it mean for a court to be activist? What does it mean for a court to show judicial restraint? Although conservatives have long complained about the activism of liberal justices and judges, in recent years liberals have pointed out that conservative judges and justices are now more likely to overturn precedents and question the power of elected institutions of government. Conservatives counter by saying they are simply returning to an older precedent that had been ignored by liberals. If both liberals and conservatives engage in judicial activism, what is the role of the concept of “activism” (perhaps judicial activism is just a term used to describe a court decision you disagree with)?