Short answers, pulled from the story.
Stanislav Petrov (1939-2017) was a lieutenant colonel and engineer in the Soviet Air Defence Forces on duty at the Serpukhov-15 bunker near Moscow on the 26th of September 1983. When the Oko early-warning system reported incoming U.S. missiles, he judged the alerts to be false alarms and did not relay the warning to his superiors, a decision credited with preventing a retaliatory nuclear strike.
The false alarms were caused by a rare alignment of sunlight reflecting off high-altitude clouds, which the satellites' sensors misread as missile launches. The satellites were in Molniya orbits that made this alignment possible. The error was later corrected by cross-referencing data from a geostationary satellite.
The Soviet Union operated under a launch-on-warning doctrine embedded in the principle of mutual assured destruction. If the early-warning system reported inbound missiles, an immediate and compulsory nuclear counter-attack against the United States was required.
Petrov reasoned that a genuine U.S. first strike would involve hundreds of simultaneous launches to destroy Soviet retaliatory capacity, making five isolated detections an illogical opening move. He also regarded the Oko satellite system as new and not yet fully trustworthy, and ground radar failed to provide any corroborating evidence.
Despite initially being praised by General Yuri Votintsev and promised a reward, Petrov was reprimanded for improperly filing paperwork during the crisis. He received no reward, was reassigned to a less sensitive post, took early retirement, and suffered a nervous breakdown.
Relations were severely strained. U.S. psychological operations involving bombers flying toward Soviet airspace had run since mid-February 1981. On the 1st of September 1983, Soviet forces shot down Korean Air Lines Flight 007, killing all 269 people aboard including U.S. Representative Larry McDonald. Cold War nuclear strategy expert Bruce G. Blair described the period as the most dangerous and intense phase in U.S.-Soviet relations.