Good question.  Unfortunately, I doubt that any leftist will be able to define it for you, ergo why that term is included in my 'Lispy Leftist List of Linguistic Lunacy'.  As far as I can gather, anyone who isn't a part of their leftist hive mind is a part of the "extreme right", whatever THAT means.
Since I am a supporter of the MAGA movement (currently spearheaded by Donald J. Trump), I have been referred to as various things, including but most certainly not limited to:
** Deplorable  (Hillary Clinton)
** Listless Vessel  (Ron DeSantis)
** Ultra MAGA  (Joe Biden and/or his handlers)
** Cult member needing "deprogramming"  (Hillary Clinton)
** Far-right extremist  (leftists in general)
** Conspiracy theorist  (leftists in general)
** Bible thumper  (leftists in general)
** Threat to "democracy"  (leftists in general)
** Racist, Bigot, Homophobe, Xenophobe, Misogynist, et al.  (leftists in general)
** Domestic terrorist  (Intelligence Branch of the SODC)