What does this passage mean from Act III of Macbeth? There the grown serpent lies; the worm that's fled Hath nature that in time will venom breed....

In Shakespeare's Macbeth, the tragic
hero, Macbeth, says this referring to Banquo and his son, Fleance.  Banquo is the
serpent and Fleance is the worm.  Banquo lies dead (he's been killed by Macbeth's hired
killers), but Fleance escaped and is still alive.  In time, Fleance will grow and,
presumably, seek to avenge his father's murder by coming after Macbeth.  But in the
play's present, Fleance is still a child. 

The idea is that
Fleance, as a child, is no threat to Macbeth--at least not in terms of raising an army
and attempting to seize the throne.  In the future he may be a physical threat, but not
in the play's present.  Macbeth doesn't have to worry about him for
now. 

Of course, in the bigger picture, Fleance's escape
signals the doom of Macbeth's hope of creating a dynasty.  According to the witches'
predictions, Banquo's heirs will rule.  Macbeth attempts to change his "fate" when he
orders both Banquo and Fleance killed.  But he fails.