The term zero filling is used to describe the process of formatting a hard drive or another storage device. Zero filling is a type of low-level format, where the entire storage device is erased by writing zeroes to every bit of the storage device. It is often used to thoroughly and securely erase all data, to make it impossible for data to be recovered later. Zero filling is a way to protect personal data from being stolen.
In addition to securely erasing data, zero filling can sometimes be used to fix a corrupted hard drive or a hard drive that is heavily infected with viruses. Some users will zero fill a hard drive before installing a new operating system, to ensure all data is erased first and that the hard drive is fully usable.