Referrer (referer)
No referrer found, meaning you probably came from a native application (Outlook, PDF reader, Word, Teams, etc.), or typed in the URL in the address bar, and not by following a link on a web page.
About referer
The Referer HTTP request header contains the absolute or partial address from which a resource has been requested. The Referer header allows a server to identify referring pages that people are visiting from or where requested resources are being used. This data can be used for analytics, logging, optimized caching, and more.
When you click a link, the Referer contains the address of the page that includes the link. When you make resource requests to another domain, the Referer contains the address of the page that uses the requested resource.
Note: The header name "referer" is actually a misspelling of the word "referrer". See HTTP referer on Wikipedia for more details.