Information about information
Metadata describes a file rather than forming its main visible content. A camera can record its model and capture time. A document editor can record an author and software version. These details are often useful for organizing and editing.
When useful becomes unnecessary
The same details can be more than you intended to share. Location may identify where an image was taken. A document property may carry a name you did not intend to publish. Different apps and sharing services preserve different fields; do not assume every destination strips them.
A small habit before sharing
Inspect a local copy, remove supported metadata, and check the visible content too. Keep the original for your own archive. Metadata cleaning is a useful step, not a guarantee of anonymity.