According to MDN:
HTTP headers let the client and the server pass additional information with an HTTP request or response. An HTTP header consists of its case-insensitive name followed by a colon (:), then by its value. Whitespace before the value is ignored.
According to Wikipedia:
HTTP header fields are a list of strings sent and received by both the client program and server on every HTTP request and response. These headers are usually invisible to the end-user and are only processed or logged by the server and client applications.
Headers can be grouped by their contexts:
Accept
Accept: text/html,application/json
Location
Location: https://example.com
X-Content-Duration
X-Content-Duration: 42
X-Forwarded-For
X-Forwarded-For: 69.42.0.0