What’s the difference between Uri.Host and Uri.Authority

Yes Brandon is absolutely correct, in layman terms

Authority = Host Name + Port No

And if URL protocol is using a default port, say port 80 for http URL, then only in that case
Authority = Host Name (Port No is assumed to be 80),

Whereas Host Name is either Domain Name or I.P Address

Example:

1. http://www.example.com/

   Authority = www.example.com
   Host Name = www.example.com

2. http://255.255.255.255:8080/

   Authority = 255.255.255.255:8080
   Host Name = 255.255.255.255