Answer: "a court order to appear"
Explanation:
a subpoena is a letter from the state, federal court-or from the Congress (the House of Representatives in the USA--(one of the three co-equal branches of the US government) informing a citizen that s/he must appear in (come to) court or Congress to answer questions at a hearing or face criminal charges in a trial.

Following this order is not an option and when someone violates it (doesn't appear or come to the court or Congress) s/he is "in contempt" of court or Congress; this is especially serious (bad) if it is the behavior of federal employees such as the President or his staff, because it signifies (shows) that s/he or they feel they are above the law; in this case, the law that gives equal power to the three branches of government.

This is particularly serious when considering that the whole point of the American democracy and it's republic is to share power-meaning to prevent power from being consolidated (concentrated) in one man or body, such as in a king or his staff.

So when the Trump administration began defying (disobeying) the law and not honoring (respecting and following) subpoenas from the Congress, it meant that the power in Washington was shifting to the President and his staff, meaning that the peoples' power in the Congress and Senate was being disrespected and thus that the president was above the law; this is how democracy begins to crumble.