In this scene, Hecate is chastising the witches for dealing with Macbeth's life and future without consulting her. She wants to be clear that the witches understand the "rules" for what they are doing.
Hecate realizes that man has free will. All of the prophesies are not dicates of fate, but rather suggestions, ideas planted in men's minds. These planted ideas grow into action, action of man's own making.
For example, a friend might suggest to another friend that a third student keeps his wallet in this bookbag during lunch. While this is in no way forcing the first person to act, the idea is set that it would be easy to steal the student's money. Perhaps the student would not have thought about stealing without the suggestion, but he still had the decision to make for himself.
Hecate goes on to say:
Shall raise such artificial sprites
As by the strength of their illusion
Shall draw him on to his confusion:
He shall spurn fate, scorn death, and bear
He hopes 'bove wisdom, grace and fear:
And you all know, security
Is mortals' chiefest enemy.
Here she plans to plant another seed, this one based on the last two lines - security is man's biggest enemy. Think about it. When we are secure, we let our guard down. When our guard is down, we are more vulnerable. The second set of prophecies shown to Macbeth make him feel secure, invincible. However, it is just this security that makes him disregard all rational information about his upcoming battle.
Research shows that most traffic accidents occur within 1/2 to 1 mile from a person's home. This is a road that the drivers know the best; they feel secure on it, unlike an unknown road or a highway. When they feel safe, they may pay less attention and then have an accident.
I hope this helps!