Sensor: Magnetic field
The experiment "Magnetic field" just gives the raw data from the phone's magnetometer.
The magnetometer is quite sensitive as it is designed to measure the earth's magnetic field to act as a compass. However, you should not trust its absolute values as they are easily influenced by magnetizations in the phone (which can even be altered by external fields) and since most phones try to recalibrate this sensor once in a while to compensate for these influences.
Do not expose your phone to strong magnetic fields!
If you accidentally expose your phone to a strong field, this may magnetize parts within your phone, leading to a constant field measured by the magnetometer. This will render the compass function of your phone useless as it becomes stuck. However, most phones will recalibrate the compass function after a while to compensate for this and the compass will start working again without any limitations - Nevertheless, you should not risk this.