Where the Power Cells Are is an in-game achievement in Run 3. It requires you to travel 2000 meters in Infinite mode without respawning. Even free respawns aren't allowed.
The achievement name is a reference to the book "Where the Wild Things Are." It's also a reference to the fact that power cells become more valuable the further you go, up until 2000 meters.
The level shown in the icon of this achievement is infinite mode level 125. The level after is IM level 166?