Superb, used in coin descriptions, is a grade modifier used to indicate a particularly nice coin for the grade. Superb usually indicates a very attractive coin for the grade. What makes a coin "superb" is mostly subjective and the opinion of the grader.
A completely intact piece with original surfaces and glaze or patina. None but the most minute chipping or corrosion. A museum quality piece. When a fragment is offered, the grading is for jus that portion present.