Referrer
The referrer is the address of the page a visitor came from, sent by the browser in the HTTP header of the same name.
A typo that became a standard
The header is spelled `Referer`, with a single "r" in the middle: a typo in the 1996 HTTP specification, never corrected so as not to break what already worked. The directive that controls whether it is sent is spelled correctly: `Referrer-Policy`.
Why some traffic stays "direct"
The referrer disappears in several perfectly normal cases: moving from an HTTPS page to an HTTP one, a restrictive `Referrer-Policy`, a click from a native application or an email client, a URL typed by hand. That traffic then falls into "direct", which is therefore not a channel but a catch-all. That is what UTM parameters are for: they carry the information in the URL instead of depending on the browser.
Referrer spam
Some bots visit sites declaring a fake referrer, hoping the address shows up in public analytics reports and earns a link there. A tool that filters bots removes most of the phenomenon; a source report containing domains you have never heard of is worth a look.