The “Smoking Gun” tape was a tape that clearly showed President Nixon involving himself in the cover-up of the Watergate scandal very early on.
The Watergate burglary had occurred on June 17, 1972. President Nixon maintained that he knew nothing about the involvement of his top aides in the Watergate scandal until March of 1973. There were many who claimed that Nixon had known what was going on prior to that date but there was no “smoking gun” that conclusively proved that he had. (The term “smoking gun” is used because it refers to the idea that someone who is caught with a smoking gun in their hand has indisputably shot that gun. It is a term that we use to refer to any evidence that proves something conclusively.)
But then, the “Smoking Gun” tape came to light. It proved conclusively that Nixon had been involved in the cover-up as early as June 23, 1972. This made it very obvious that Nixon had been lying about his participation in the cover-up. Only three days after the tape came to light, Nixon resigned.