'Filament Slip' is the term we use to describe an issue with feeding filament. Inside the extruder (filament feeder), there is a wheel with a rubber tyre (index wheel) which rotates as the filament runs past, and this is used to measure exactly how far the filament has moved using an optical light gate. If there is a discrepancy between what the firmware has told the filament to move, and the measured move from the index wheel, a filament slip error is raised.