Using a simple little command to copy multiple files to one location, this trick basically works by merging a compressed file with an image. In other words, you can save your top-secret files inside an image by saving them both into one file.
The final result is an image file that also contains the secretly stored compressed file, which you can access in 2 ways: opening the file normally will show the image and hide the rest; otherwise, choosing to open the file with an archiver will reveal all the secrets (which only you can know about).