The medium grind stage shapes and smooths your stones. Medium grit is finer than coarse grit so the medium grit will shape the stones a little but mostly it works on the surfaces of your stones starting the smoothing process.
When changing over your tumbler from coarse to medium grit, be sure to clean your stones and barrel well. Use an old toothbrush to get any grit out of cracked and indented areas. At this time determine if you need to add any more filler to your load to keep the barrel 3/4 full. It is still alright to add rough crushed stones at this stage for more filler. Again use a tablespoon of grit per pound of rock and run for seven days.
The grit on the left is 150-220 (medium) grit. The grit on the right is 600 and finer (fine) grit. The numbers or "range" in the grit is determined by the number of granules per inch if they were placed end to end.
The image on the left has been tumbled with 60-90 grit. The image on the right has been polished with the 150-220 grit, and is ready for the fine grit stage.