To explain what a response header is, it's good to know what a request header is.
Every user surfing the web who is opening a link or a website is doing a request with his or her browser. The browser is the client. The client is doing a request. In this request is a component called request header. In this header operating parameters can be defined. The client is sending the request message to a server. The server sends a response message back. A part of this response message is the response header. It's containing further information for the client. Like noindex, MIME-Type, Canonical or compression.