Stringing
Stringing is when thin strands of filament are left on a print, usually between separate parts, because plastic keeps flowing as the nozzle moves.
Why it matters
Strings spoil the surface and take time to clean off. Prusa names two main causes: a printing temperature that is too high, and incorrect retraction settings.
Retraction pulls the filament back slightly before the nozzle travels. Prusa suggests keeping retraction length to a maximum of 2 mm on direct-drive models and increasing retraction speed. It also suggests lowering the nozzle temperature by 5 to 10 °C.
Material plays a part. Prusa lists stringing as a drawback of PETG, and its profiles use higher retraction and a lower nozzle temperature for it.
Moisture can also cause it. Prusa suggests trying a different spool if stringing continues, because moisture in the filament makes it worse. This links to hygroscopic materials, which absorb water from the air.
A spool left open in a humid room can take on moisture. Store filament sealed, and dry it if you suspect it is wet.
Start with the temperature and retraction changes, then check the spool.