A referer is the URL of the web page that forwarded a visitor. In affiliate marketing, a referer is a piece of extra code behind the link (url parameters) by which the advertiser can tell that a visitor came from an affiliate network.
Read more about the referer clicks here.
Fun fact
While 'Referrer' is the correct spelling, we use 'Referer' with one 'r' in our systems - a misspelling born from an early internet typo that stuck around and became the actual standard when used in this context.