A Kickstarter Exclusive item is something that will only ever be available through this Kickstarter campaign (and as promo items at conventions etc.) and will not be generally available as a retail product.
Something that is a KS ONLY UPGRADE (and not also a Kickstarter Exclusive) may at some future point be available as an additional purchase in retail. My stance on this subject is that significant gameplay features (such as characters and game boards) should be available to all players if they wish to acquire them. At this point there are no plans to have the Elemental characters available for purchase in retail, it is possible that they will be made available at some point.