The meter that appears on the levels once you have managed to match bubbles three times in a row is called Streak Meter. When you fill the meter by matching bubbles six times in a row, you’ll get a special bird boost. If you fling a bubble without popping anything, the Streak Meter will reset back to zero.
In case you miss a shot and do not want to lose your bird boost, you can fill the meter instantly with your coins. Just tap the meter or the bird to open the purchase screen.