Many people probably think that "High Crimes and Misdemeanors" is just an 18th century way of saying “felonies and misdemeanors.” But this interpretation is mistaken.
The term does go back to English law and was used to remove officials in colonies. In fact it was a fairly common term in 18 the century. Most of the framers of the constitution knew the phrase well.
Officials accused of “high crimes and misdemeanors” were accused of offenses as varied as misappropriating government funds, appointing unfit subordinates, not prosecuting cases, not spending money allocated by Parliament, promoting themselves ahead of more deserving candidates, threatening a grand jury, disobeying an order from Parliament, arresting a man to keep him from running for Parliament, losing a ship by neglecting to moor it, helping “suppress petitions to the King to call a Parliament,” granting warrants without cause, and bribery. Some of these charges were crimes. Others were not. The one common denominator in all these accusations was that the official had somehow abused the power of his office and was unfit to serve.
High Crimes and Misdemeanors - Constitutional Rights Foundation