Discrimination is the ability for the detector to miss out certain metals. Without discrimination a metal detector would give a beep on everything even silver paper and the user wouldn't be able to go far without filling his pockets or pouches full of rubbish..
A favourite saying of ours is that discrimination should be used with discrimination. Pull tabs, for example, are made of aluminium a highly conductive metal. The discrimination on a metal detector would have to be turned up quite high to make the machine ignore them but at the same time gold could never be found having a lower conductivity than aluminium.
Most detectors have a 0-10 scale, users will find their own preferred settings depending on what land they are searching. On a personal basis, if on ploughed land we use no discrimination, we want to find everything and farmland doesn't suffer from too much scrap. If on parkland we use a setting of about 2-3, our machines reject gold at about number 4.