These are the Pseudo-Header Fields defined in the new HTTP / 2.
HTTP / 1.x used the start-line message to represent the target uri, request method, response code, etc. All HTTP messages are either a request from client to server or a response from server to client. The two types are distinguished by their start-line , which is request-line for requests or status-line for responses.
request-line = method SP request-target SP HTTP-version CRLF status-line = HTTP-version SP status-code SP reason-phrase CRLF
You can read more in RFC7230 section 3.1
But for this purpose, HTTP / 2 uses special pseudo-header fields starting with the character ':' (ASCII 0x3a). These pseudo-headers are strictly defined. There are pseudo-header request fields and response pseudo-header fields. Request pseudo-header fields :method :scheme :authority :path .
The ":authority" pseudo-header field includes the authority portion of the target URI. authority = [ userinfo "@" ] host [ ":" port ]
More details in RFC7540 .
In HTTP / 1.x, it is equivalent to the host header. RFC7540 mentions some measures for backward compatibility with HTTP / 1.x
"To ensure that the HTTP / 1.1 request string can be reproduced accurately, this pseudo-header field MUST be omitted when translating from an HTTP / 1.1 request to request at the beginning or in the form of an asterisk. Clients that generate HTTP / 2 requests directly SHOULD use the pseudo-header field ": authority" instead of the host header field. An intermediary that converts the HTTP / 2 request to HTTP / 1.1 MUST create a host header field if it is not in the request by copying the value of the header alias field ": authority "