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 to do so.