Laptop GPU reballing

What is meant by reballing?
Assemblies which fail due to bad BGA connections can be repaired either by reflowing, or by removing the device and cleaning it of solder, reballing, and replacing. Devices can be recovered from scrapped assemblies for reuse in the same way
 
What does reballing a GPU do?
Reballing is the process in which the GPU is removed, and the tiny solder balls on the bottom of the graphics card are replaced with a better, lead-based solder. It's then replaced back on the motherboard and reflowed to form proper bonds with the motherboard.