Definition
A slicer commonly retracts before a travel move, then advances the filament again before the next printed segment. In M83 relative mode this may appear as E-0.8 followed later by E0.8. In M82 absolute mode it appears as the E position moving backward and then returning.
Retraction is not a promise that melted plastic is pulled cleanly back into the nozzle. It changes pressure in an elastic system. Excessive distance or speed can grind filament, draw softened material into a cool zone or delay the next extrusion; insufficient retraction can leave strings.
What matters in practice
Paired operation
A later prime normally restores the amount that was retracted before new material is counted.
Mode-aware
Negative numbers alone do not identify retraction in absolute E mode; the delta from the previous E value does.
Machine-dependent
Direct-drive and Bowden systems often need different settings.
Example in G-code
Relative retract and prime
M83
G1 E-0.8 F2400
G0 X80 Y80 F9000
G1 E0.8 F2400Pressure is relieved before travel, then the same commanded amount is restored.
What this viewer can show
The viewer excludes a retraction and its matching re-prime from deposited-filament totals. That avoids inflating material use every time the slicer crosses a gap.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.