"High crimes and misdemeanors" means official misconduct that is so severe as to subvert the governmental administration of the national interest.
But it does not have to be.
-Secretly letting a foreign dictator annex the U.S.
-Only releasing federal funds to states that vote for you.
-Declaring "The Purge" and firing U.S. attorneys who indict anyone for crimes on that night.
But that isn't relevant to whether it is an impeachable offense.