Ground balance (ground mineralisation)...
The ground contains minerals and metals in trace amounts, the mineralisation of the ground varies from place to place with the gold fields up North and beaches being particularly bad; however these trace amounts are enough to interrupt metal detectors from giving you a clear signal and thereby 'hiding' potential finds. What ground balancing does is measures the amount / type of mineralisation in the ground and compensates for it, effectively cancelling it out so you can hear potential targets easier. The better the ground balancing the 'deeper' the detector can scan without interruption from mineralisation. With some detectors ground balancing is manual, with some it is automatic, and yet with others (higher up the product range) you get 'tracking ground balance' which means the detector continuously monitors the ground for changes in mineralisation and compensates for it, leaving you to detect in peace.