Rift Keystones (or RKs) are small objects that drop randomly or are awarded for completing all five bounties in a given Act in Adventure Mode. RKs have no function on their own and are only useful to open up a Nephalem Rift. As that is a very useful purpose, they are quite valuable objects.
RKs stack up in the inventory like other commodities in the game. These objects were BoA when seen in the Blizzcon 2013 demo, and it's assumed they will remain soulbound, akin to the various materials required for the Infernal Machine event.