It’s an informal title for an appointed office in the White House, one of the executive agencies (e.g., the Office of Mangement and Budget), or a cabinet-level department. It’s used as shorthand and to convey power. There is always a (longer) real title, but sometimes (as in freido’s example) the real title doesn’t convey the influence that this person is supposed to carry. Another example: The “Manufacturing Jobs Czar” is really the Commerce Department Assistant Secretary for Manufacturing. Normally, Commerce Department Assistant Secretaries are not influential positions.
The first time I heard it being used was in the 70’s, but it could extend back earlier than that. samclem could probably answer the first use question.
There are cites in the mid-1800’s in the US using the term to show that a person had great power, but not the “drug czar” type of thing we use so much today.
That type of word formation first appears in the 1930’s, i.e. “beer czar” used of a state official, 1942–‘the war production czar’ etc.
So it’s still rather old, at least 75 years or so. And it IS a U.S. originated thing, as far as cites that I can find.