Zerofree finds the unallocated blocks with non-zero value content in
 an ext2, ext3 or ext4 file-system and fills them with zeroes
 (zerofree can also work with another value than zero). This is mostly
 useful if the device on which this file-system resides is a disk
 image. In this case, depending on the type of disk image, a secondary
 utility may be able to reduce the size of the disk image after
 zerofree has been run. Zerofree requires the file-system to be
 unmounted or mounted read-only.
 .
 The usual way to achieve the same result (zeroing the unused
 blocks) is to run "dd" to create a file full of zeroes that takes up
 the entire free space on the drive, and then delete this file. This
 has many disadvantages, which zerofree alleviates:
  * it is slow;
  * it makes the disk image (temporarily) grow to its maximal extent;
  * it (temporarily) uses all free space on the disk, so other
    concurrent write actions may fail.
 .
 Zerofree has been written to be run from GNU/Linux systems installed
 as guest OSes inside a virtual machine. If this is not your case, you
 almost certainly don't need this package. (One other use case would
 be to erase sensitive data a little bit more securely than with a
 simple "rm").