Mother was a title given to female elders or womanly leaders in various matriarchal cultures such as the Witches of Dathomir or the Tarasin species.
The "Mother" (sometimes Clan Mother, "Matron" or "Matriarch") honorific was also applied to leaders of Dathomiri witches clans, including certain Nightsister covens. Such leaders included Gethzerion, the founder of the Nightsisters, Mother Talzin, a Nightsister Shaman during the Clone Wars, and Mother Rell fo the Singing Mountain Clan.
The earliest Clan Mother of Dathomir was the banished Jedi Knight Allya, who literally sired the first witches and taught her daughters the ways of the Force through the lens of magicks. A Dathomiri Clan Mother was the heir of Allya's authority over the clan members, known as Sisters.
For a mere Sister, a convenient way for the echelons of power was to kill the current Mother(similar to the Sith's typical method of succession). If the assassination attempt was successful, the murderer was considered worthy of taking her victim's place.
On Cularin, a "Mother" was the eldest woman of a Tarasin irstat. This woman was considered a leader and wise-woman, usually having served for several years as an Irstat-Kes.