Basically a door frame that you walk through, it beeps if you have anything metallic on you.
while it is mainly used at airports and cruise terminals to stop people with guns and bombs, schools are now using it to stop students from bringing in iPods and phones
metal detectors are commonly accompanied by X-ray machines to scan your bags
Kid 1: Dude did you bring your iPod?
Kid 2: nah, I would've but the fucking metal detector beeped and they took it from me