This is stage three of a series of trophies tied to the collection of Power Cells (101 in total):
The final 101 value will not come into play until the end, and you will see why. If you go into the start menu, you should be able to see brief descriptions from every area along with how many Power Cells can be found in each. A note of interest would be to make sure that you stay in the area to collect all you can (Precursor Orbs, Power Cells, Scout Flies) before moving on.
*Note: Some instances will require you to back-track at later times through your gameplay. Examples would be turning on the Blue Vents in the Forbidden Jungle or the Yellow Vents in Snowy Mountain.
**As all areas come with listings for how many Power Cells are found in each, I will avoid listing unnecessary numbers.
The reason you need to collect all 101 Power Cells is to finish the game with 101% completion. Once you complete the story, you will find a Precursor Gate that has slots for 101 different Power Cells, and Samos and Keira suggest you return when you have them all. If you make sure you collect all of the Power Cells before taking the elevator in the Dark Citadel, all you will have to endure is the boss battle and the credits to be treated to the final ending.