File slack is the disk space from the end of a file till the end of the last cluster used by that file. A cluster is the minimal portion of disk space used by the file system. For example, if you create and save a long document that fills up 75% of a cluster, and then you delete it with a standard delete command, then the data will still be available in that 75% of the cluster, but the cluster itself will be available for future reuse. If you then create and save a short document that fills up only 50% of that same cluster, then the 25% left over from the previously deleted document will still remain in the cluster.

The data in file slack (or slack space) is invisible by simple windows file editors (e.g., Notepad, MS Word), but it is easily read by other special utilities. So it's important to wipe file slack space to have total confidence that all traces of your data have been permanently deleted from the disk.