Hecate, who doesn't appear in Macbeth until Act III, scene 5, is the "mistress" or head of the witches. Her name derives from Greek mythology, where she was a goddess associated with witchcraft, the moon, magic, and also with protecting the home and helping it to prosper.
In Macbeth, she is angry with the three witches for interacting with Macbeth without her knowledge or participation. She implies that they don't know what they are doing and that they have wasted their time prophesying for Macbeth. She is also angry because they are helping a man that she says is only interested in himself, not the witches: she calls him a "wayward son ... spiteful and wrathful," who "loves for his own ends, not for you."
This angry speech suggests that the three witches have been trying to help Macbeth with their prophecies to curry favor with him. Hectate implies that strategy won't work, because Macbeth is entirely self-interested.
With her entrance, the tables definitively turn against Macbeth. If the other witches were trying, however misguidedly, to help him (though their intentions are not clear), Hecate is purely out to wreak damage on him. Perhaps because she is angry at the other witches' interventions or because she doesn't like Macbeth, she is not going to protect Macbeth's home or bring him prosperity. She says she will spend the night preparing a "dismal and a fatal end" for Macbeth. Leaning into the mythological roots of Hecate, Shakespeare envisions her traveling to the "corner of the moon" to collect a magic "drop" that will confuse Macbeth with a sense of false security and bring him to his doom.
Hecate is a dangerous, frightening figure. She is malevolent and angry at the other witches, and, in comparison to them, ruthless in her intentions towards Macbeth.