The filling of traditional meditation cushions is usually either kapok tree fiber or buckwheat hulls. Some use polystyrene beads to avoid their tendency to become hard when sat upon.
Buckwheat hull filling will generally make your cushion about twice as heavy as one filled with kapok tree fiber, which is extremely fine and light. When you sit on a cushion filled with buckwheat hulls, it will mold to your seating position and not compress much further over time.
Kapok is softer but less firm overall. Some prefer it for its lightweight and softness.