A metal detector is an electronic instrument which detects the presence of metal nearby.

This app measures magnetic field value using the built-in magnetic sensor that is built into your device The magnetic field level(EMF) in nature is about 49μT( micro tesla ) or 490mG (milli gauss); 1μT = 10mG.
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
Usage is simple: Open this app on your device, and move it around. The magnetic field level will constantly fluctuate. That's it!