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It's the preliminary stage, makes it a formal investigation so they have a pretext to issue subpoenas etc. They may decide to hold a vote on impeachment which (I think) requires just a simple majority. If that passes, the president is impeached (as was Clinton). Then the Senate is supposed to hold a kind of trial with a judge, prosecution and defense followed by a vote on whether to convict which requires 2/3rds to pass. Given the current Senate it's pretty unlikely they'll hold the trial let alone vote to convict - afaik no president has ever been convicted following impeachment.
QUESTION - is the impeachment investigation the preliminary stage before impeachment or is it the actual impeachment?
Hope that makes sense