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.
I believe that Fleance has escaped and Banquo has been killed, so all in all Macbeth tried to change his fate but he couldn't, so now we know that Fleance will become king according to the witches.