Content Type
In plain terms
A header declaring the media type of a message body.
Going deeper
The Content-Type header tells the recipient what format the payload is in, so it's parsed and rendered correctly. It specifies the body's MIME type.
Under the hood
Drives parsing and content negotiation; mismatches cause rendering or security issues.
Examples
application/json, text/html, multipart/form-data, HTTP headers, APIs.
Think of it like...
Like a shipping label stating exactly what kind of goods are inside.