A spice harvester is a large, heavy, mobile factory designed to harvest the Spice Melange. It is dropped by carrier ships (known as Carryalls) onto spice fields. These machines would then harvest and process the spice off the top of the desert floor.
Because of the rhythmic sound they make, they are regularly eaten by sandworms. That's why 2 or 3 ornithopters (called spotters) flew in the sky above the spice harvester, watching for wormsign (an obvious sign that a sandworm is approaching). Upon detecting wormsign, the harvester would be picked up immediately to clear it out of the way. A sandworm can be large enough to swallow the harvesters.
When Muad' Dib asked Liet Kynes if they would see a sandworm while viewing the spice harvesting, Kynes replied that they always come when they hear a spice harvester.