Keepers are elves with minds that are close to impenetrable.
When something is too important to write down, the Council or another organization gives the secret to a Keeper to remember and protect. They are mostly Telepaths. It is the Keepers' job to remember things.
At the beginning of Book One: Keeper of the Lost Cities, Quinlin thought that Sophie Foster is (or would be someday) a Keeper due to the fact that the Black Swan made her with an impenetrable mind, which is the ideal for keepers.
The opposite of a Keeper is called a Probe.