Temple Run 2 Power Ups are an equitable buff that will give you advantages in certain situations. You meter on the top left will build up when you collect coins during a run. Once your meter is full double tap anywhere on the screen to use. You can only equip 1 at a time though you can find all of them randomly through out a run so having them all built up to max in the abilities menu will help you out a lot. Each are the same across all characters and when you unlock one it will be unlocked for everyone. You can raise there strength in the abilities area and doing so will also upgrade the same for every character.
For more info check out the Temple Run 2 Guide Wiki
Shield – This will be your first available power up and will save you from 1 mistake for a limited time. After the time is up the shield will disappear. When compared to the similar skill Boost which will make you invincible and catapulting you forward a short to long distance very quickly, Shield will offer much less protection, but a better chance to grab those chests and gems.
Coin Bonus – When you reach a certain distance the value of coins will go up letting you to get more faster. Each upgrade in the abilities menu stays though the description changes. Example: 2nd upgrade says values double at 1000m, 3rd upgrade say they triple at 3000m, you now have both the second and third upgrade. You must get this as soon as possible if you think your gonna be in it for the long haul.
Boost – Makes you completely invincible and sends you running at great speed forward. Will help get you to your Coin Bonus faster and more often. Traveling at fast speeds can cause you to miss chests and gems.
Score Bonus – This will increase your score every time you fill your meter or when you find it in a run. Helpful for tying to get the best score in the world…possible.
Coin Magnet – Gives you a magnetic force that will attract every coins you pass no mater where you are. This can be very useful at high speeds when your along ways in and can really only focus on not dying.
Gem Bonus – Not sure yet, just started out. My guess is that it will increase the power of any power ups found while running like adding gems in the top right corner. If so this would go great with the Pickup Spawn ability which increases the frequency of coming across P Ups during a run.