The IRS measures aircraft acceleration and senses attitude throughout the flight to compute groundspeed, aircraft track and, of course, position.
i.e. The aircraft begins at zero knots at the departure gate at a known position. When the aircraft starts moving, groundspeed is computed from the sums of all major and minor accelerations in all directions (over the duration of the flight).
Unfortunately, the system is not 100% perfect, so tiny errors in the measuring system accumulate over time. This may result in the IRS thinking that it is still moving at the end of a flight and/or the IRU thinking it is not where it really is.
Residual groundspeed is the speed error recorded at the end of a flight (aircraft not moving). If the error is too large, then there may be a problem with the IRS.
Your instrument displays may show this residual groundspeed error, especially on aircraft which have no GPS input into the Flight Management system.
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NSEU