In these lines, Hecate states her intentions in regard to Macbeth. Consider what she says in the entire passage, lines 25-33:
I'll catch it [a vap'rous drop hanging on the moon] ere it comes to ground:
And that distilled by magic sleights
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
His hopes 'bove wisdom, grace, and fear:
And you all know security
Is mortals' chiefest enemy.
Hecate will use her supernatural powers, her magic, to create spirits to appear to Macbeth and present false illusions. They will confuse Macbeth, leading him to act in ways that defy fear and wisdom. They will give Macbeth a false sense of security, causing him to scorn even death.
The passage foreshadows the appearance of the three apparitions that speak to Macbeth in Act IV. When he hears that "none of woman born" can hurt him and that he will never be defeated until Birnam Wood comes to Dunsinane, Macbeth accepts these prophecies. They give him the false sense of security to which Hecate had alluded. He has been tricked into defying fear and common wisdom, leading to his death.
I agree with mshurn's answer. The witches are planning to tell throw Macbeth into a tailspin and the line about security being mortal's chiefest enemy it the base of their plan. They are going to make him so confident in his newfound crown that his judgment will be off. They know what is going to happen and if they make him feel secure in that his crown and the "invincibility" that comes with it, then they know that they will be able to get him, or let someone else get him. His guard will be down. Coupled with his greed and ambition, Macbeth is in big trouble here!