The thing is, when you get rid of a file by emptying the Trash, you’re just removing an access point (icon/directory listing) to the file. It doesn’t erase the data, it discards it. This means that the data itself isn’t actually deleted, making the file recoverable via third-party apps.
Erase files securely with Shredder.
This means that the file data isn’t just discarded, but written-over. The access point removed, and the data is rewritten (read: scrambled), making the file irrecoverable. When you erase something with Shredder, you cannot restore it — it’s a tool for really getting rid of files.