Minecraft wiki entry for Bone_Meal, associated text: Bone meal can be used on the sides or bottom of a grass block, as well as on a grass block with plants already on top of it.
Regardless of if it succeeds in fertilizing, bone meal makes green sparkles. However, like smoke from a torch, the sparkles appear only if the player has particles set All or Decreased.
In real life, bone meal is used as a fertilizer to provide nitrogen and phosphorus to plants.