When you are in Fringo, click the Power Up Box and it will give you this page.
It tells you how many of each power-up you have - click each and learn what they do.
3 lightning bolts fill the meter. When you fill the power-up meter, use it right away. If you have the meter 2/3 full, try to pick a fruit that only has one lightning bolt to maximize power-ups in the game.
Make sure you visit Fringo once a day to get free power-ups.
More info here: Fringo Guide