Data about data. The information attached to a file or communication that describes when it was created, where, by what device, and sometimes by whom — without being part of the content itself. A photo's metadata includes GPS coordinates, device model, and timestamp. An email's metadata includes sender, recipient, time, and server path — even if the content is encrypted.
In plain language: the hidden information attached to everything you create digitally, which can reveal your location, device, and identity even when the content itself is private.