In short, REX is a middle-man currency that is purchased with real money and can then be converted into item store credits (although, as the post states, it's a better deal to directly buy the credits). The twist here is that before it's converted to credits, REX can be sold to other players on the auction house. This effectively gives players with in-game money ways to obtain store credits and players with excess real money ways to inflate their in-game piggy banks.
The post also explains the much simpler topic of gifting store items to other players. You probably have it figured out just by reading the previous sentence.
Finally, last night we told you about RIFT's promo this weekend. According to a tweet this afternoon, that promo is live early -- in fact, RIFT is freely playable for former subbers right now.