The witch means that her toad, her familiar, is calling.
A witch’s familiar was an animal that was kind of like her special friend. Witches kept their familiars close to them. Elizabethans were a superstitious bunch. They believed that witches often used an animal, often cat or a toad, to help them cast spells. Thus, the Second Witch here is literally being called by her familiar, a toad or paddock, to help her cast the spell. First Witch has a cat as her familiar. His name is apparently Graymalkin, which literally means “gray cat.” This whole exchange makes a bit more sense then.
Where the place?
Upon the heath.
There to meet with Macbeth.
I come, Graymalkin!
Paddock calls. (Act 1, Scene 1)
In other words, First Witch got called by her familiar, her cat, and then Second Witch got called by her familiar, a toad.
The witches are here just to mess with Macbeth. They have agreed to meet him after the “hurlyburly's done,” in other words, after the chaos of the batter is over. Then they will give him their three prophecies and start messing with his head!
Macbeth is really quite gullible. He falls for the witches, hook, line, and sinker. They are really a sight. By Banquo’s description he is not sure if they are human, and if they are men or women.
You seem to understand me,
By each at once her chappy finger laying
Upon her skinny lips: you should be women,
And yet your beards forbid me to interpret
That you are so. (Act 1, Scene 3)
Yet they predict that Macbeth will be first Thane of Cawdor and then king, and he likes that. Banquo they tell will have sons that are king. He is suspicious. He tells Macbeth not to listen. Macbeth, however, is overtaken by greed. He decides that he deserves these honors, and as soon as he learns from Duncan that he is not to be king, the chaos ensues.
The witches really drive the story, meeting up with Macbeth again to tell him more prophecies before the final battle. They do not take part in any of the violence themselves though. All they do is light the fire. Macbeth provides the violence. He is the one who kills Duncan, Banquo, and Macduff’s family. They would not have been able to cause any trouble if he had not been an ambitious man to begin with. They knew what they were doing when they chose him.