The word is older than analytics dashboards. When a browser follows a link, it may tell the next site which page it just left. That previous URL is the referrer. The header is spelled Referer, missing an r, because of a typo in the original spec. Dashboards quietly correct the spelling.
What you can learn from it
The hostname is the useful part. example.net linked to you. A path on that hostname sometimes tells you which article. A list of those hostnames, with counts, is a referrer report. MapMyVisitors stores the referrer on the visit, so that list is available next to country and page.
You learn which sites introduced people. You do not learn the Google query. You do not learn which of three links on the same page was clicked, unless you tagged your own links. Tagging is UTM tracking, and it is something you add. The browser will not invent it.
When it is empty
No previous page was sent. The person typed the address, used a bookmark, opened an app that hides the source, or followed a link with rel="noreferrer". Those visits sit in direct traffic. An empty referrer is a normal row. It is not a sign that tracking is broken, unless every visit is empty, including ones you just clicked from another site you control. That second case means something on the path is stripping the header.
A limit worth remembering
The referrer is whatever the browser chose to say. It can be absent, shortened to the origin only, or missing the path. Read hostnames and counts. Do not rebuild a click map you were never sent. The one-line definition lives in the glossary as referrer.
