Not sure what machine you own, but here goes:
The SENSitivity button/knob controls how strongly your detector picks up the returned signals bounced off metal objects in the ground...this controls how deep your detector will hunt (to a point)....Adjusting this too high is the cause of most false signals encountered by new hunters...only turn it as high as you can without the detector chattering when no target is present...
The DISCrimination button/knob does exactly what its name implies....i discriminates out certain metals you may not want to dig, such as iron, nickels, pull tabs, and screw caps, also zinc pennies...Remember that GOLD reads in the same range as iron/foil, pull tabs, screw caps, and in some cases, zinc....I have had large class rings ring in as copper/1c, so turning this up can help and also hinder your finds, depending on where and what you are hunting for...The higher you turn this, the more your missing...read your instruction manual to fully understand these...
HH,
GoGoGopher