:It means the amount of thrust it's producing. Which correlates fairly well with the fuel consumption (but not perfectly due to changes in burn efficiency at different thrust levels). Right now the Merlin 1D is throttled to about 75-80% of full thrust at most, because they've been running it in what we might think of as "safe mode" until they were sure it could survive full thrust. A very slightly modified version of the engine is due to debut in a few months (1D+) that will be rated to throttle over that full range. It will be regularly throttled to near 100% on standard missions. —