Bone meal is an item that is used as fertilizer or as a white dye.
It can be obtained by crafting it from bones. A bone yields 2-6 pieces of bone meal depending on the type of bone you use. See this table on the bone page for the actual amounts.
Shapeless Crafting 2-6
You can also craft it from dirt and nitre:
Shapeless Crafting 2
Or grind up a bone block:
Shapeless Crafting 8