Thread is the term given to an organism brought to Pern by the Red Star from the Oort Cloud. Thread falls upon Pern for a period of 50 years out of the Red Star's 250 year orbit, creating the fundamental division of Pernese history of Passes (50 year periods) and Intervals (200 year periods - except for the Long Intervals, which are 450 years).
Thread is a voracious, though simple, organism. It consumes any and all organic material until it dies from overeating, though a small number appear to enter an unknown second phase.
Without the Dragons to fight Thread, Pern would be nearly uninhabitable. No crops would be possible during a Pass, and even during the Interval, so much land would be damaged by Thread that it would take many decades for cropland to be restored.
Thread can only be killed by cold, fire, or water. During a Threadfall over the ocean, fish eagerly consume drowned Thread, allowing fishermen to capture large catches after Threadfall. Life would probably be possible without the Dragons, subsiding on the produce of the oceans during Threadfall, but it would be far less comfortable.