Usually when people mention clearing the trash out they mean a systematic removal of all metallic items ( trash ) in the ground so that any good targets that were masked by the trash can now be detected. Since much of the time these trash metal targets may mask or " hide "....or blend with , any good targets that may be there , fooling the detector , once they are gone the chances of finding something good can at times be significantly improved. Doing this can take a lot of work depending on how much trash is there but can also be worth the effort at times. Since the aim is to remove all or most of the metal trash , you want to use very little if any discrimination , dig every target , and clear as large of an area as possible. Clearing the trash wont guarantee anything but you are atleast removing all the known obstacles that can keep the good stuff hidden.