Magnetometers can be used to detect orientation with respect to the Earths magnetic field. Basically, like a compass! We can tell which way is North, and thus correct for motion calculation errors and 'absolute orientation'
Good stuff! But, magnetometers have to measure a very small magnetic field of 35-65 uTesla, in a world full of magnets. And there's some offset when they are manufactured and pick and placed.
Of all the sensors that need calibration, magnetometers are the most essential to calibrate! Unless you're detecting strong magnets, there's no way for a magnetometer to work unless you perform a hard iron offset calculation. Once this is done, you will get rid of any strong magnetic offset values and be able to find magnetic North!