Imagine a briefcase where different keys open different compartments. One key might show your grocery list, another shows your real important documents. If someone forces you to open it, you can give them a "fake" key that shows harmless stuff.

This is called "deniable encryption" - you can honestly say "there's nothing important here" when using the decoy password.

And it is impossible to understand whether any one of the compartment is a random content or an encrypted one.