What it means is that when you compress into a ZIP (lossless, as we mentioned), you will not lose anything of the original file. The mp4 file is structured/built in a way that reads it as a media file, whereas a ZIP file takes all that data, and crams it into one small space that is less "open"
You can think of this as something that folds up, like a chair. The only way it is useful is when it's "open" but even when it is closed all of the "data" is still there.