Crisis on Infinite Earths is a 12-part comic book series published by DC Comics in 1985. It was devised as a way for the company to simplify its then 50-year-old continuity.
The story effectively wraps up the continuity of the company's fictional universe, and serves as a launching point for its new continuity. Some characters received a full reboot, such as Wonder Woman and Superman. John Byrne handled duties for Superman's new launch in the 6-part Man of Steel mini-series published a few months after Crisis.
Elements of Superman's history noted as "pre-Crisis" usually refer to those stories published before Crisis on Infinite Earths (unless more specifically cited as pre-Infinite Crisis, another company event meant to again simplify continuity since Crisis on Infinite Earths).