It's a big metal ring, and the blue wobbly bit is a wormhole that will disintegrate you when you step in and will then take you to another Stargate on another planet where it puts you back together again.

...I can't WAIT to have someone explain that to our Top Gear trio.

You just know someone's going to make a joke about the dangers of sending Richard through it, since he can't really afford to lose any bits, at least with Jeremy they've got a foot or so to play with ;)