A Reverend Mother was both a Bene Gesserit title and a class or level held by a woman of the Bene Gesserit who has completed her training.
At the end of their training, the Bene Gesserit undergoes a ritual involving a significant amount of one of a class of awareness spectrum narcotics. The drug enters her body as a poison. If she survives this, by a conscious transformation of the drug on a molecular level to a non-poisonous substance, she gains the memories of all the preceding Reverend Mothers in her line, resulting in great knowledge and power. The particular narcotic used by the lines of Reverend Mothers on Arrakis is contained in the Water of Life (Dune), the liquid exhalation of a sandworm produced at the moment of its death by drowning.
This ritual would be fatal for normal males. In an extensive breeding program, the Bene Gesserit have tried to produce a male who could survive the transformation, the so-called Kwisatz Haderach. The final, though unexpected result of their program was Paul Atreides.
In the novel Dune, we discover that the Fremen have their own independent tradition that produces wild Reverend Mothers from the Bene Gesserit perspective, called Sayyadina. These Sayyadina go through the spice agony and have access to their ancestral female memories; they however lack the other skills that the Bene Gesserit have developed.
Paul Atreides' sister, Alia Atreides, was accidentally transformed into a Reverend Mother as a preborn while inside the womb of Jessica Atreides. The effects of this transformation shape the action in the novels Dune Messiah and Children of Dune.
In the novel Chapterhouse: Dune, it is discovered that the descendants of the Jews, who have still retained their ancestral traditions, have also learnt the Bene Gesserit secret; and have started to produce their own Reverend Mothers.