Secure delete means to overwrite data in a file when the file has been deleted from your computer. Using the Windows operating system, when a file is deleted it is simply removed from the file system table, so the file exists on the hard drive until other data overwrites it. During this time the file could be recovered.
A number of software tools exist that claim to securely delete files by ensuring the sectors on the hard drive that the file occupied prior to delete is immediately overwritten with other data. Programs that overwrite the data multiple times are considered more secure. There are similar utilities, called space wipers, that will write data to all free space on the hard drive to remove all traces of any file that has been deleted on the Windows system.