Rock salt being ground into salt powder.
The quern is a structure that is used to grind grains into flour, and making powders.
Constructing the quern requires a base made of 3 smooth stone blocks combined in the crafting grid, and a handstone made from 3 raw stone blocks with a stick. Place the quern base on the ground, and then right click it to open the quern interface. Place the handstone in the top slot, and the material to grind in the left slot. Exit the quern interface; you should now see the material floating above the handstone. Right-click the handle on top to grind the material, an animation of the handstone turning will play. Right-click the quern to collect the resulting powder/flour.
Aside from milling grains into flour, the quern is also used to grind minerals for fire clay, fertilizer and rock salt, and others.