http://www.pcgamer.com/civilization-6-rise-and-fall-trailer-details-golden-and-dark-ages-emergencies-and-more/

The latest Civilization 6: Rise and Fall trailer takes a break from introducing new leaders and nations to provide an overview of the new systems that will be ushered in by the expansion, including Great Ages, Loyalty, Governors, Enhanced Alliances, Emergencies, and new Wonders and Units. 

The primary addition, Golden and Dark Ages, are temporary changes to a civilization that last for an era. Both can dramatically alter the state of the game and will force players to adapt their strategies accordingly, but while Golden Ages are obviously preferable, Dark Ages have upsides too: Golden Ages come more easily when emerging from a Dark Age, and they also enable Heroic Ages, which grant three Dedication bonuses instead of just the single one that comes with a Golden Age.