What is a Cookie header?
Sends previously stored cookies back to the server that set them.
Cookie is what makes a website feel stateful across requests - it's how sessions, shopping carts, and login state survive between page loads. A bug in cookie handling (wrong domain, wrong path, an expired session cookie not being refreshed) tends to manifest as intermittent, hard-to-reproduce login problems for real users, which is exactly the kind of issue that's much easier to catch with a monitor that maintains a real session across repeated checks than with a one-off manual test.
Common use case
A browser automatically re-sends a previously issued session cookie on every request to the same site, keeping a visitor logged in across page loads without resubmitting credentials.
Example
Cookie: session_id=a1b2c3d4; theme=darkHistory
Cookies originated at Netscape in 1994; the request-side Cookie header was formalised in RFC 2109 (1997) and later RFC 6265 (2011).
Did you know?
The original cookie specification came out of an early, largely unrelated project to let an e-commerce site remember whether a visitor had already filled a shopping cart - a use case that still dominates cookie usage decades later.