This screen means to keep iPhone connected so it continues charging.
Use a high-power USB port such as the one on your computer, or use your iPhone AC adapter.
Don't use the USB port on your keyboard because it is a low-power port and iPhone will not charge.
Make sure you are using USB to charge. Accessories that use FireWire to charge are not supported with iPhone.
If you see the red part of the battery image flash three times and then the screen goes black, it means that iPhone is not receiving power to charge.
clipped from: www.apple.com