From a microchip datasheet:
4.5.2 OSCTUNE REGISTER
The internal oscillator’s output has been calibrated at the
factory but can be adjusted in the application. This is
done by writing to the OSCTUNE register (Register 4-1).
The tuning sensitivity is constant throughout the tuning
range. The OSCTUNE register has a tuning range of
±12.5%.
So, you use the OSCTUNE register to adjust the internal oscillator and calibrate it, if you need accurate timing. Obviously, you need an accurate time source to compare to. Shift is the amount of adjustment that is applied.