What was important about the "smoking gun" recording? The tape revealed President Nixon's involvement in the Watergate scandal. The tape repudiated the FBI investigations into the Watergate break-in. The tape disclosed the reason for White House firings. The tape uncovered the identity of Deep Throat.

Answer: The tape revealed President Nixon's involvement in the Watergate scandal.

Context/details:

The first break-in by burglars was in May, 1972, as persons connected to the Nixon reelection campaign broke into the Democratic National Committee office in the Watergate complex of buildings in Washington, DC. They were planting wire-taps on the Democrats' phone lines, and also stole copies of documents. When the wire-taps didn't work properly, they broke in again (in June) to try to fix the surveillance devices, but they were caught.

Nixon's role in Watergate was especially in his efforts and those of members of his staff to cover up what had happened.

In July 1974, the White House was compelled to turn over subpoenaed tapes over to Special Prosecutor Ron Jaworski. As reported by Politico in an article (8/5/2018) remembering the release of the tapes:

One of those tapes was the so-called "smoking gun" tape, from June 23, 1972, six days after the Watergate break-in. On that tape, Nixon recorded an Oval Office meeting with H. R. Haldeman, his chief of staff. During their exchange, Nixon agreed that administration officials should approach Richard Helms, the director of the CIA, and Vernon A. Walters, its deputy director, and ask them to ask L. Patrick Gray, the acting director of the FBI, to halt the bureau's investigation into the Watergate break-in because it was a national security matter. Jaworski believed that Nixon, in so agreeing, had entered a criminal conspiracy to obstruct justice.

Ultimately, the Watergate affair brought down the Nixon presidency. He resigned in order to avoid impeachment. And the whole affair made Americans more distrusting of government.