These lines are spoken by Hecate (one of the three witches) right before Macbeth decides to go and beg them for more information. Quite simply, these lines mean that death comes when a mortal least expects it. You see, Macbeth has just seen the ghost of Banquo and panics as a result. In his panic he says the following:
I will tomorrow, / And betimes I will, to the weird sisters: / More shall they speak;for now I am bent to know, / By the worst means, the worst. (3.4.157-160)
The witches know that Macbeth is coming and promise to give him just that "security" that they have warned us about. In fact, the lines that you quote do a lot to prove that the witches have it in for Macbeth all along because of the intense security their words give him in the next act. Consider telling Macbeth simply to "beware Macduff" because "none of woman born shall harm Macbeth" and that he cannot be killed "until Great Birnam Wood to High Dunsinane Hill / Shall come against him" (4.1.79,90,104). Macbeth's next words prove how secure he is: "That will never be" (4.1.106). Ah, but you underestimate Shakespeare! Not so fast, Macbeth. Not so fast.