Most metal detectors work by emitting electromagnetic waves and 'reading' the signal as it bounces back to it. However, in every day life there are electromagnetic signals most everywhere, from overhead electric cables to other metal detectors near by! So, in order to not get lots of readings from 'bogus' sources you can use a function called noise cancelling. This is either manual or automatic depending on your brand of detector. On automatic you would simply push a button and the detector will run through several 'channels' of available frequencies and use the one with the least interference from other electromagnetic sources.