What do we mean by “above the law”?

I’m reading an Atlantic piece on the Mueller report as impeachment referral, and I stop at this paragraph:

But there is another, simpler way to understand Mueller’s report. A footnote spells out that a criminal investigation could ultimately result in charges being brought either after a president has been removed from office by the process of impeachment or after he has left office. Mueller explicitly rejected the argument of Trump’s lawyers that a president could not be guilty of obstruction of justice for the conduct in question: “The protection of the criminal justice system from corrupt acts by any person—including the President—accords with the fundamental principle of our government that ‘[n]o [person] in this country is so high that he is above the law.’”

It’s a weird idea, that (figurative) height could place someone “above” the law. (The statement is that it doesn’t, but that requires the concept to exist first.) Lots of people think it is or should be the case, of course, but it’s still a weird idea. It’s the other way around, really: the more power and status and clout you have, the more constrained by the law you should be, because you have more power to do massive harm.