The Progression bar is a bar on Trove. it's purpose is to serve as a way of watching how far you have progressed with your Mastery Level. You'll notice as you play Trove you'll gain Mastery Points. If you get 500, you'll earn a reward on weekdays, and more on weekends.
There is also a "progression bar" on your Hotbar which shows you how much experience you have earned / need to level up.