Extra Time Boosters is like a powerup, it will help you pass those difficult levels if you decide to use it. Extra Time Booster only appear when you run out of time on a level and you have not met the goal. If you decide to you use the Booster it it will give you an extra 7 seconds.
You can get Extra Time Boosters in 3 ways:
1. Buy them from the shop
2. The Daily Reward bonus will sometimes give you them to you.
3. In-game Surveys (if available) will always give you a Booster.