Answer:
When Nixon's White House tapes demonstrated that he had authorized the payment of "hush money" to those involved in the Watergate break-in, and it became clear that he was about to be impeached, Nixon resigned.
There was much talk of a criminal prosecution after he left office. Gerald Ford, the Vice President who had taken over as President at Nixon's resignation, was constantly asked about the subject. He decided that the country could not afford to go through such a trial, which would likely last years, so extended a full pardon to Nixon.
Nixon retired to New Jersey and stayed out of the limelight. But his reputation, particularly in the area of foreign affairs, was somewhat revived in the 1980s, chiefly by his writings on the subject.