The access keys are ter'angreal linked to the massive Choedan Kal, the two greatest sa'angreal ever created. As tapping the Choedan Kal unaided is invariably deadly because of the immense quantities of the One Power available through them, and the fact that carrying a gigantic statue is not feasible, the access keys were created both to act as buffers between the user and the sa'angreal while being portable.
Three access keys have been found, two by Rand in Rhuidean and one alone, though there is a possibility that more may exist. The keys are shaped as small-scale replicas of their respective sa'angreal.
Rand and Asmodean fought for the male access key found in Rhuidean, with Rand emerging victorious. Lanfear also arrived in search for the female ter'angreal, but concluded that it was either not present or destroyed, although Rand recovered it intact.
This one known workable female key was destroyed during the cleansing of saidin, along with the female Choedan Kal. The male key was taken into possession by Cadsuane Melaidhrin after the cleansing. A damaged version of the female key was (and presumably still is) in one of the displays within the Panarch's Palace in Tanchico, first found when Egwene al'Vere went Dreamwalking to search for the Black Ajah. The Broken Female one is extremely dangerous to channel into, and even hurt Egwene just by touching it in Tel'aran'rhiod.
After Rand banished Cadsuane from his sight, he had her room searched and recovered the male access key, which he then kept with him at all times, nearly using it for disastrously destructive purposes on several occasions.
After traveling to the top of Dragonmount and realizing his madness, Rand draws every ounce of saidin that he can through the access key, and then pushes the power back into it, shattering the key and the Choedan Kal deliberately.