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.
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.