什么是Referrer？
The URL of the page a visitor came from, sent by the browser and used by analytics tools to attribute traffic to its source.
The referrer is the URL of the page a visitor was on when they clicked through to your site, passed along by the browser in the Referer header (the misspelling is original to the HTTP spec and has never been fixed). Analytics tools read it to group traffic into sources: a referrer of google.com becomes organic traffic, news.ycombinator.com becomes a referral.
Referrer data is leakier than it looks. Most sites send only their domain rather than the full page URL (the strict-origin-when-cross-origin default), native apps and email clients often send nothing, and HTTPS-to-HTTP navigation strips it entirely. Every one of those missing referrers lands in direct traffic, which is why direct is best read as "unknown" rather than "typed the URL."
For links you control, UTM parameters sidestep the whole problem by putting the source in the URL itself. For links you do not control, the sharing that happens in chat apps and DMs shows up only as dark social.