I think that the action you are referring to is that the House of Representatives was starting to discuss impeaching President Nixon. In fact, they had gone past just discussing it. In response to this, President Nixon resigned.
In order for a president to be removed from office, he has to be impeached by the House and convicted by the Senate. President Nixon never actually got impeached by the House. However, the House Judiciary Committee had voted to recommend that Nixon be impeached. This made it very likely that the whole House would vote to impeach.
Nixon realized he was likely to be convicted and removed from office so he resigned.