The third episode has recently aired, and in recent weeks fans have been wondering what a weaver is whenever it has been referenced.
According to the Wiki page for the All Souls trilogy, weavers are a type of witch who possess the ability to create new spells.
Ordinary witches have to rely on resources such as their family, coven grimoires or Books of Shadows which once belonged to practitioners of the Higher Magics.
Weavers are obligated to use their abilities in order to create new and unique spells, which allow for witches in general to have more complex magics at their disposal.
Weavers can also use inherited gifts and basic elemental magic or spells without any training, as these skills do not require creating any new spells.