What does Macbeth mean when he says, "The worm that's fled Hath nature that in time will venom breed, No teeth for the present"?
We're in act 3, scene 4. Fleance, Banquo's son, has fled, escaping the murderous fate of his father. The first murderer enters and describes how he cut Banquo's throat. Macbeth automatically assumes that he's also finished off Fleance. Unfortunately for him, that's not the case:
"Most royal sir,
Fleance is 'scaped."
Immediately, Macbeth becomes scared, riddled with doubts and fears over Fleance's escape. So long as he's at large, he's a threat; he's almost certain to seek revenge for his father's murder. Macbeth is nervous; he needs reassurance that at least Banquo's really dead. To which Banquo's assassin replies:
"Ay, my good lord: safe in a ditch he bides,
With twenty trenched gashes on his head;
The least a death to nature."
So, Banquo is the "grown serpent" lying in a ditch. Fleance, however, is a "worm." Macbeth is engaging in word-play here. In Shakespeare's time a worm could refer to a dragon or a snake. It was also used to describe, as it is today, a worthless human being. As far as Macbeth is concerned, Fleance has shown himself to be a worm by running off. He has "no teeth for the present," no guts to stand and fight. But, in due course, he'll develop the courage (the "venom") to come back and take revenge. This is just what Macbeth's afraid of.
He means that Fleance escaped the murderers (he's the "worm that's fled"). Fleance is supposed to be the one from whom kings will come ("in time will venom breed"). Macbeth says that Fleance may in time hurt them as he becomes king or becomes the father of kings, but that he's too young and insignificant right now to bring him harm ("no teeth for the present"). Macbeth has become very paranoid. His paranoia began when he killed Duncan and heard voices saying that he would no longer sleep and when he tried to say, "Amen", he couldn't. From that moment on, he has become increasingly paranoid to the point where he had his best friend, Banquo, killed. He wanted Fleance killed because of the witches' prophecy that Banquo would be the father of kings.
Macbeth speaks that line as part of speech that's a bit longer in Act III, Scene IV. He's talking about the fact that Banquo is dead, but, unfortunately, his son Fleance got away. He refers to Fleance as a worm because he thinks of him as treacherous and likely to turn on Macbeth, but, because of his youth, doesn't think Fleance is likely to be a problem for a while. That's the part about having "no teeth for the present." Pretty poetic response for a man hearing a report about paid assassination.