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.

Going on the information it had that the President might be involved in wrongdoing or criminal activity, Congress launched a series of hearings.  These hearings on Watergate compelled members of the President's Cabinet to testify to Congress officials about what they knew, when they knew it, and to what extent the President was involved.  The testimony was damaging to the President in that there was a greater sense of understanding about how President Nixon conducted himself in office and how he viewed the office in terms of it superseding the law.  The hearings also revealed a network of audio tapes that confirmed what President Nixon knew and when he knew it.  Congress immediately moved for a vote of impeachment of the President, before which the President resigned.