This is the meter that appears under your Spins count meter. The progressive leads to a reward if you fill it up. For more details, tap the progressive meter to see its quest!
The goal is on a timer, so be careful -- when the timer hits zero, your progress will reset, but don't worry, the reward won't change until you've earned it. The more rewards you earn, the more fun rewards that will be added to the reward progression! This is shown with your ""Tier"" -- a higher tier means a longer reward track with better rewards toward the end. Finish a Tier, and you get to do the track again, but a new reward is added to the end of the track, and your Tier increases!