Glossary
Wet filament
Wet filament has absorbed moisture from the air, causing popping sounds, tiny bubbles and weaker, rougher looking prints.
Filament, especially PLA, PETG, TPU and especially nylon, absorbs moisture from the surrounding air over time, particularly in humid climates or when spools are left uncovered between prints. That absorbed water turns to steam the moment it hits the hot nozzle, and it has to escape somehow while the plastic is being extruded.
The signs of wet filament are fairly distinct once you know them: audible popping or crackling sounds coming from the hotend, a rougher or slightly foggy surface finish instead of a smooth one, tiny bubbles or pinholes on visible surfaces, and prints that seem weaker or more brittle than usual for no obvious reason.
Drying filament, either in a dedicated filament dry box or an oven set carefully to a safe low temperature for that plastic, usually restores normal print quality. Ongoing storage in a sealed container with desiccant prevents the problem from coming back. A common mistake is drying filament once after a bad print, then leaving the spool out in the open afterward, which lets moisture creep back in within days.