Explain Macbeth's second encounter with the witches.

Macbeth visits the witches a second time because he's growing increasingly anxious and wants some reassurance about his future. After committing two murders, he realizes that he is on a path of no return. And although the witches' prophecies are always ambiguous, the possibility of a sense of security drives Macbeth back to them.
It's interesting that he manages to both insult the witches and question their activities once he arrives:

How now, you secret, black, and midnight hags?What is ’t you do? (4.1.48–49)

Of course he knows what they are up to, and that is precisely why he has come. He is told to "Beware Macduff." This is one of the most straightforward things the witches say. They then tell him that he cannot be harmed by any man "of woman born." This seems to rule out every man on the planet, giving Macbeth a false sense of safety. He is then told that he will not be conquered until Birnam Wood marches to Dunsinane. Again, this seems impossible, so Macbeth doesn't question it. The news seems to be in his favor, and he hears what he wants to hear: Don't be afraid. It's all going to work out okay.
Of course, things definitely don't end up okay, and each of the prophecies comes to fruition in the end. Macbeth curses himself in the final lines of this scene:

Infected be the air whereon they ride,And damned all those that trust them! (4.1.153–154)

Macbeth himself trusts their words, and he therefore damns himself.