Hydrophobic coating loosely means a thin water proof coating on a material that otherwise isn’t waterproof.

Example: the previously mentioned popcorn bags are paper based but with a thin plastic based coating. (Historically we used to use wax for this kind of thing but in the modern day it’s almost always plastic)

Not that containers made of glass, ceramic, or metals that don’t corrode don’t need and typically don’t have hydrophobic coatings.