The judicial Power of the United States shall be vested in one supreme Court and in such inferior Courts as the Congress may from time to time ordain and establish. The Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behaviour, and shall, at stated Times, receive for their Services, a Compensation, which shall not be diminished during their Continuance in Office.
The Constitution establishes a single Supreme court, and then allows Congress to establish other inferior courts. Judges hold office as long as they behave themselves. This is not defined in the Constitution itself, but over the years has been interpreted to mean that they get to hold the office as long as they want to unless they are impeached for a crime.
The idea here is that the other branches of government cannot remove a Federal judge because they don’t like the rulings. They can only be removed for criminal activity. (Ya know… like a bribe???)