Revealed at the Google IO conference, Cardboard is a scored, flat-pack box that you fold into set of cardboard goggles that hold your phone; an accompanying software package uses your phone's screen and accelerometer to create stereo-optical VR images in the manner of the Oculus Rift. It's a delightfully simple and elegant concept, and Google has published plans for making your own. You need cardboard, a set of cheap lenses, a magnet, velcro and a rubber band.
What are the magnets for and can I use cardboard without them?
The magnet is used for clicking inside of demos. When you pull and release the ring, your phone's magnetometer detects changes in the magnetic field. Chrome experiments currently don't use the magnet, but you will need it to try the Cardboard Android app.
Not all phones will be able to detect magnet pulls. You can use Cardboard without the magnet by sticking copper tape along the side of the viewer and having it come into contact with the phone screen. When you tap twice on the copper tape with your finger, the phone will react as if it had detected a magnet pull.