In this image, Seany selects a cuboid of unknown dimensions, presumably 1x1x1m, then he expands it 1,000m in all 4 directions, 50m upwards, and 50m downwards. So the cuboid is a total of 2,000x2,000x100m, and contains 400,000,000 blocks. He then runs the smooth command, which smooths out jagged parts of the terrain, for example smoothing a pyramid would turn it into a rounded hill. The integer following the command indicates that the command will be run 100,000 times, one after the other. So he runs a command affecting 400,000,000 blocks 100,000 times. This works out to a final total of 40,000,000,000,000 (40 trillion) blocks being processed.