What is the number under your letter grade rating at the end of a match?

Hi everyone,

Does someone know what the number is under your letter grade rating at the end of a match? For example, I recently got an S grade with +1571 at the bottom of it. Is that exp gained or something? Or is that Elo or something else? If it is XP, then I guess it's a waste if your already champion mastery 7?