Stringing
Stringing is the thin wispy strings that from on some prints. The nice round stuff that happens when a print fails is spaghetti.
There are lots of reasons stringing happens. Start with how it happens. You are drawing a line. You have a nozzle full of molten plastic, it is being pushed out of the nozzle, the new filament being pushed in by the extruder is raising the pressure in the nozzle and forcing the molten stuff out. You stop the line and move the nozzle to a new place to start drawing again. When you stop the molten plastic in the nozzle is still attached to the molten, but rapidly cooling, line you just finished. As you pull the nozzle away surface tension keeps the molten plastic attached to the line, also since the pressure in the nozzle is high more of the molten filament wants to come out. This leaves a string behind.
What can you do about it. A major cause is that the molten plastic in the nozzle is at pressure, so you can lower that pressure. The easiest way to lower it is to pull back on the new filament. Retraction. There is how far and how fast. In a bowden setup the filament is bent and pushes at the side of the bowden tube, so you need to pull back further to effect the nozzle. Direct drive removes most of the stuff between the extruder and hot end so the pull back is less. How fast you pull back also affects it. These things depend on your printer and the filament you are using. Settings used by someone else are just a starting point and you may end up with different ones.
Moisture in the filament can also cause stringing. When the filament melts the water in it turns to steam and increases pressure in the nozzle. If this happens while you are drawing a line you can get little pimples of filament. If it happens as you pull the head away the increased pressure can cause stringing.
Temperature can also affect stringing. A filament will usually string more at higher temperatures. It is not as simple as always lowering temperature as lower temperatures have worse interlayer adhesion. You have to balance two different things. A print with pointy bits that does not need great interlayer bonding might do better at a lower temperature than a print of a box that needs high interlayer adhesion.
If you have stringing problems you can easily dry the filament to remove one of the causes. New filament, straight from the sealed plastic bag with a little bag of desiccant inside, can be wet. Part of the manufacturing process involves the freshly extruded filament being put in a water bath to cool. Before the spool is sealed it can sit on a shelf in a humid warehouse. New filament is not necessarily dry.