Hygroscopic
Hygroscopic describes a material that readily absorbs water molecules from the air around it.
Why it matters
Prusa notes that most filaments used for FFF printing are hygroscopic to some degree. Water trapped in the plastic turns to steam in the hot nozzle, which shows up as bubbles, uneven layers and a rough surface.
Some materials are far more sensitive. Prusa says nylon can absorb water up to 10% of its weight if stored badly, and that wet TPU and other flexible filaments become hard to print. Even PETG should be dried before printing, according to Prusa.
The fix is in two parts. Dry the spool before you print, then keep it in a sealed box with silica gel or desiccant. Prusa advises moving a freshly dried spool straight into the drybox so it does not reabsorb moisture.
For nylon, Prusa’s guidance is at least 4 hours at a temperature below 90 °C. Check the manufacturer’s label for other materials, because safe drying temperatures vary.