Condensate line
The small drain pipe that carries water away from your AC or furnace.
When your air conditioner or high-efficiency furnace runs, it pulls moisture out of the air, and that water has to go somewhere. The condensate line is the small PVC or vinyl pipe that drains it away. Algae and gunk clog it over time; when it backs up, water overflows the drain pan and can stain ceilings or flood a closet. Flushing it (often with a little vinegar) once or twice a year prevents that.