A brand new, higher capacity battery pack for GameBoy Advance SP handheld consoles.
Third party/unofficial (as in not made by Nintendo) but fully compatible and has a higher capacity than the original battery (850mAh vs 600mAh).
Easy to replace - just open the battery compartment and swap out the old one.
The battery compartment is held in place by a small Philips screw - a screwdriver for this is included with this new battery.