In the book Macbeth, act 4 scene 1, what are the witches doing when this scene opens?

The witches complete their magic spell and summon forth a series of apparitions. The first is an armed head that warns Macbeth to beware the Thane of Fife (Macduff). The second apparition is a bloody child, who tells him that "none of woman born / Shall harm Macbeth" (96-97).