Welcome to Right Hand Rule!
This app is used as a way to interface the magnetic sensor inside of your phone.
Just put your phone up to anything and it will tell you the amount of Teslas that object emits.
The higher the tesla value, the more magnetic the object.
The percentage sign is there to show the amount of magnetism it is computing as opposed to the maximum amount of magnetism your phone can detect (Usually 10mT)
Also, the app becomes more red as the magnetism grows!