A Kelda is the leader of a Feegle clan. She is the one with "the knowing of the reading". She keeps her Clan in order, makes decisions, and is effectively their head of state.
Feegles are much like bees, in that they have extremely few females, and the Clan will seem to consist of all males. This is because a Kelda "might be blessed wi' only one daughter in her life, but she'll have hundreds and hundreds o' sons". Each Clan is made up of the hundreds, sometimes thousands, of sons of the Kelda.
A daughter cannot run her mother's Clan, so those females born in one Clan must leave and find another one in need of a Kelda. She cannot marry a brother, and a new "Big Man" cannot be brought in because the men of the clan would not know him, and, therefore, would not respect him. Usually she takes a small retinue of her brothers with her to the new Clan. Once there, she chooses one of the warriors for a husband.
A Kelda is guarded fervently by all the Clan, even temporary Keldas such as Tiffany Aching. In her case, this means reading her diary, following her everywhere and the kind of constant surveillance that some governments have naughty dreams about. Tiffany is less than thrilled about this.
Upon her death, a Kelda is buried in the mound with all the previous Keldas. Those of her brothers who still remain will then usually ask permission from the new Kelda to return to their original home.