The format in this respect is the size of the image sensor, which is much larger than traditional "full frame" sensors. When you pack the light sensing pixels on a sensor closer together you lose image quality, so a larger sensor allows you to pack more pixels onto the sensor while producing better quality images.
However, medium format cameras are expensive and big. You can easily pay £10 - £20.000, while professional full frame cameras are "only" measured in thousands. This makes the X1D a "bargain" at £7.188. In terms of weight, the mirrorless makes the camera light enough to carry around your neck weighing less than a kilogram.
I would love one of these, but unfortunately this bargain is outside my budget.