Why did Macbeth visit the witches a second time? Where did he find them this time and what were they doing?

In Act IV, Scene 1, Macbeth visits the witches for a second time because he is afraid he might lose his position as king. Now that he has become king, Macbeth is paranoid that others will discover that he killed Duncan and kill him or dethrone him in some other manner. Since the witches were the ones who told Macbeth that he would become king to begin with, he now returns to them to see if they can foresee whether he will lose the throne or who he should avoid or kill to ensure he remains king.

When Macbeth finds the witches the second time, they are standing over a cauldron to make a potion and cast a spell. This scene has a couple of the most famous lines in Macbeth: the witches say, "Double, double toil and trouble;/ Fire burn and cauldron bubble" over their potion, and when Macbeth is about to appear on stage, the Second Witch says, "By the pricking of my thumbs/ Something wicked this way comes." This indicates that the witches know Macbeth killed Duncan.

Once Macbeth is onstage, he asks the witches to tell him more about his future. To do this, the witches conjure three different apparitions. The first tells Macbeth to beware Macduff, who is Thane of Fife. Seemingly conversely, the second apparition tells Macbeth that no one "of woman born" can harm him. This causes Macbeth to think he does not need to fear Macduff after all, though he says he still plans to kill Macduff. The third and final apparition says that Macbeth will not be defeated until "Great Birnam wood to high Dunsinane hill shall come against him." Macbeth takes this literally to mean his defeat will come when the trees in Birnam wood come against him, which he knows is not possible. Thus, Macbeth's conversation with the witches and the apparitions makes him feel far more secure about remaining king.