It means that the black hole already has that much mass in it. There is no known limit to the mass gain of a black hole. It is unlikely to encounter as much matter to grow say twice again it's size since space is so vast and matter so sparsely distributed over it. But if you introduced the entirety of matter in the universe to a single point and it didn't have velocity away from that point it would all merge under gravity to form the biggest black hole possible. Empty space around it would loose all meaning since there would be nothing in it. Also supermassive black holes host the galaxies around them not the other way around.