DetailedBatteryUsage is a tweak from developer Hamza Sood that enables Apple's hidden battery usage menu in iOS 8. From that menu you can see your device's battery life graphed out and view detailed information on the power used by individual applications.
For those times when a simple percentage isn't enough.
This unlocks the hidden battery usage menu Apple uses internally during development (found in Settings->General->Usage->Battery Usage). It contains much more detail than the standard menu, check the screenshots for an example.