What does the passage in "Macbeth" that begins, "There the grown serpent lies . . . ." mean?

Macbeth is talking on one side to the murderer, who has just told him that Fleance has escaped, but that Banquo is dead...

MURDERER: 
                       Safe in a ditch he bides,
With twenty trenched gashes on his head;
The least a death to nature.

MACBETH:
                                   Thanks for that.
There the grown serpent lies; the worm that's fled
Hath nature that in time will venom breed,
No teeth for the present. Get thee gone.

Macbeth's metaphor is simple: Banquo is the "grown serpent", the snake grown to full size. Fleance is at the moment, only a baby snake, the "worm", and he's fled. However, he has a natural predisposition to grow into a poisonous snake with real teeth, though he has none for the present.

In short, Fleance isn't dangerous yet. But he will be. And when Macbeth says "there... [Banquo] lies", he's simply saying that the fully-grown serpent is lying in a ditch: where the murderers leave Banquo's corpse.

The metaphor echoes another of Macbeth's in the play: "we have scorched the snake, not killed it", he says to Lady Macbeth. Snakes, in this play, are difficult to kill, slimy, escaping: and, of course, terrifying and venemous.