What is the rinnegan?

It is an eye technique, like that of the sharingan, possesed by Pein, or otherwise, Nagato. It is the most powerful eye techniques, with the sharingan and the byakugan.Rinnegan also allows the user to sense chakra, including an otherwise hidden barrier. Find out more about the rinnegan on the Naruto Wiki.
I don't know much about about it but i can tell you somethings. Pain (leader of Akatsuki) AKA Nagato has a special pair of eyes. When his other 6 pains are loose and causing mayhem. They basically are able to see what their comrades are seeing. EX. You attack a Pain from behind while another is looking at what you're doing. The one your attacking sees through his teammates eyes and blocks the attack. Basically you would have to fight them 1 on 1 to win.