The grindstone is a handy block that you will use in Age 0 prior to acquiring a Horse Power grindstone. It allows you to grind items, such as ink sacs or bones, into items such as black dye or bone fragments (and eventually, bonemeal), respectively, by way of manual labor. It is not particularly fast, and requires several right-clicks to use, but it will get the job done. You can extract ground items by shift-right-clicking the grindstone with both hands empty. It requires six (6) stone and one (1) stick to craft, found in the JEI interface.
Similarly, the Horse Power grindstone accomplishes the same tasks, but is, obviously, powered by a horse (or other animal, such as a buffalo, cow, llama, etc.). All that you need to do is place the grindstone two (2) blocks above the ground, then clear a three (3)-block radius around the grindstone for the animal to walk. Then, just get a lead and attach an animal to the grindstone. You can then place items that you want to be ground into the grindstone, and it will do it automatically. It is generally advised to put a hopper underneath the grindstone so that you don't have to stand and remove the ground items constantly.
Note: TRY TO HAVE ALL HORSE POWER LEADS IN ONE CHUNK. CROSSING CHUNKS WILL SOMETIMES CAUSE THE LEADS TO COME OFF. - as of SevTech build 3.0.8.