"Referer" is a field in the HTTP request headers that contains the previous page that linked to the requested page. This misspelling of the word "referrer" was present in the original proposal for the field in the HTTP specification, made by Phillip Hallam-Baker. This spelling was then widely implemented by the software of the time, and solidified by RFC 1945 which documented the HTTP/1.0 protocol as it was being contemporarily used.
More details on this can be found on this field and its etymology on its Wikipedia entry.