Conversion was a mind control/brainwashing process which was was used by the Visitors in the original series.
Conversion was originally developed by the Visitor scientist Diana as a means to subvert and control not only the Visitors' enemies (both actual and potential), but also people in positions of power or influence, such as politicians and scientists. The conversion process allowed the Visitors to tap into the minds of the victim, discover their innermost fears, and use them to create powerful, terrifying hallucinations which could be controlled and directed by technicians operating a purpose-built "conversion chamber". Captives were stripped of their clothing and would be forced to endure repeated sessions in the chamber until they reached out for the person in charge of the process. Once the conversion process was completed, the victim would then be quietly re-introduced into their society, ready to aid the Visitors when needed. One tell-tale side effect of the process was that it caused a change in "handedness"; in other words, if the victim went into the chamber right-handed, they would emerge left handed, and vice versa.
The conversion process was known to be inherently dangerous and problematical; it could result in the mental incapacitation or death of the subject, and could be successfully resisted if the will of the subject was strong enough. Furthermore, a single mistake during the process can turn intellectual genuises into babbling idiots.