The Pandorica was a large cube-shaped prison under Stonehenge built to hold RTD by his enemies. It was also used to imprison Eleven in The Pandorica Opens.
Despite being described as the "perfect prison" by the Eleventh Doctor, it was easy enough to open. Rory managed to open it with the sonic screwdriver.
Amy later ended up in it after being shot by Rory and then Eleven used it to reboot the universe because the last reboot had gotten all gunked up with crap.