"The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors"
That any president or judicial official who is deemed unstable or commits certain crimes (like bribery, Treason etc...) can be impeached
Impeachment: "The process of charging officials in the executive and judicial branches with wrongdoing and bringing them to trial" (in other words it is saying that someone is doing something wrong, and they should be put on trial to be removed from office)
Impeachment process: starts at House of Representatives. Then the president (or any other government official who is going to be put on trial for impeachment) goes to the Senate for trial (which the Chief Justice presides)