Android: Phones with AMOLED displays have a unique, battery-saving trait: the fewer pixels are activated, the less power they consume. Pixel OFF helps you take advantage of this by only activating a mesh of pixels.
The app can disable a grid of pixels of varying density, depending on how much battery you want to save. Your phone will look mostly the same, if a bit darker. However, because your display is using half as many pixels (or less), your battery usage from the display should decrease. Many phones these days have so many pixels, you'll hardly notice the difference.
Pixel OFF also includes Tasker support so, if you don't want to leave it on all the time, you can choose to only enable the grid based on certain Tasker conditions. For example, when your battery life reaches a certain floor, or only when you're away from home.