The signature

Imagine a sealed box with one word on the lid: FRAGILE.
The word changes nothing inside. It tells you how to treat it.

Every JPEG works the same way. It opens with a few bits that say "a photo starts here."
A program reads that label and knows: paint the rest as a picture.

Hand those exact same bits to a different program, one expecting text, and you get nonsense.

The bits never changed. Only the agreement did.
That agreement is where the meaning lives.