HTTP headers are standardized and contain usefull informations.

• Host: the target of the request.
• Referer: Address of the web page that generated the query.
  
• User-Agent: Browser used to log in.
  

Custom headers can be freely added. Here 'X-MyHeader: value':

GET / HTTP/1.1
Host: localhost:8001
User-Agent: curl/7.58.0
Accept: */*
X-MyHeader: value

Add a custom Header with curl : -H 'header: value'

$ curl -H 'X-MyHeader: yoloforpresident' http://localhost:8001