They are called Q-Link pendants and you wear them as a necklace. They are sold on a website found here and other places such as Amazon.
I've seen them before. Lindsey Lohan wore one (I don't know if she still does but I recall seeing a picture of her wearing one) and several other celebs (Madonna, Dennis Hopper, Salma Hayek) and several athletes - especially golfers - are reported to have worn them. They come in a variety of colors and shapes in prices ranging from $99 to $3099 for the high-end gold tag pendant and are also available in bracelets. You can buy them for your pets, as well.
What are they supposed to do? Q-link pendants and bracelets are supposed to neutralize the "harmful effects" of electromagnetic fields (EMFs). Electromagnetic fields are created by electric appliances, microwaves, power lines, etc. Many people believe that EMFs cause cancer although there are no studies showing a connection between EMFs and cancer. Q-link doesn't say anything about cancer but claims that EMFs can cause stress and fatigue.
What are they made of? Dr. Ben Goldacre opened one up to find out. Turns out that that they contain a circuit board that isn't connected to anything and a zero-ohm resistor (a resistor that has no resistance) that is not connected to anything. He ran a variety of tests and could find no evidence that it was, in fact, doing anything - which makes sense as a circuit board needs to be attached to something to do something.
So... we have a product that does nothing that is sold to do something that cannot be proven and costs a pretty penny to purchase - but hardly anything to produce. Sounds to me like we have the makings of a scam here. Despite endorsement by Dr. Oz (or perhaps because of it), I cannot recommend you spend any money on these products. There is no evidence that the problem they claim to neutralize even exists and no evidence that these products do anything at all.
Image found here