Sifter is an early-game station that separates the powder-like blocks (like Sand, Loose Silt, Dirt, etc.) and returns any useful materials that are found inside. Most blocks cost 0 pixels, and the materials within them can be very valuable, so this is very beneficial to do.
Sifter processes 1 block every 0.33 seconds. Later in the game it gets replaced by Powder Sifter, which is faster and yields better results.