What does the "erase free space"?

Wiping free disk space When you delete a file, Windows removes the reference to that file, but doesn’t delete the actual data that made up the file on your hard drive. Over time, this data will be overwritten as Windows writes new files to that area of the drive.
For More Information Please Refer:
https://www.ccleaner.com/docs/ccleaner/using-ccleaner/wiping-free-disk-space
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