Ducol, files are allocated in
blocks. If you have a very small file, say containing just text "Hello, world!", it would still occupy a full block in storage. If the file is larger than one block, only the last one may be partially used. Block size of 512 bytes is probably the most common nowadays, but it does vary a lot.
Some filesystems do use
tail packing, where such partially used "blocks", containing the "tails" of files, are combined.
Slack space sanitizers simply seek to find partially used blocks, and write zeroes to the unused parts, without modifying the actual information stored. That's what their name comes from: "slack space", because it is not used to store any useful information right now, and "sanitizer", because the intent is to clear it to zeros (just in case it at some point in the past contained some information).