Stringing, also known as oozing or "hairy" prints, is a common defect where small whiskers of plastic are left behind as the print head moves. At its heart, stringing is a fluid dynamics problem: molten plastic is a viscous liquid, and gravity combined with residual pressure in the nozzle will cause it to leak out unless specific countermeasures are taken.
Retraction is the primary mechanical tool used to fight stringing. When the printer finishes a line, the extruder motor reverses, pulling the filament back. This creates a small amount of negative pressure that sucks the molten plastic back into the nozzle. The two main settings to tune are retraction distance and retraction speed.
Temperature is the second most important factor. Every increase in nozzle temperature decreases the viscosity of the molten plastic, making it runnier and more likely to ooze. Furthermore, wet filament can cause "bubbling" as water turns to steam in the nozzle, forcing plastic out uncontrollably. If settings are perfect but stringing persists, check for damp filament.