You're awarded a Chest once you complete your progress bar. These Chests are tailored to your playing activity.
However, you get Special Chests independently of the rewards program. You don't need to complete the progress bar to receive these Chests. You can earn them as part of a special offer, in specific games, or by completing some tasks.
Like any other Chests, they have an expiry date (which is displayed when you receive them). You can open them from the 'My Rewards' tab.