One of imazing feature in new smartphone is the magnetic sensor. It allow to find magnetic onde.
This app use this technologie to find metal it search for magnetic onde. If there is any metal in the area the strength of the magnetic field should increase. Sometimes this app used to find electrical wires in walls (like a stud finder) and iron pipes in the ground.
If this app does not work properly it may mean that your phone's sensors are damaged broken or non-existent. To initialize the sensors point your phone up towards the sky and move it in a figure 8 pattern.