What does Laertes mean when he says this? And how does it advance the plot? Why, as a woodcock to mine own springe, Osric. I am justly kill'd with mine own treachery.
Laertes is presented as a noble and honorable young man. He only decides to use poison on an untipped foil because of his passionate desire for revenge and because King Claudius has encouraged him to kill Hamlet by whatever means possible. When Laertes and Hamlet exchange foils during their duel, Laertes is forced to continue the match or else show his knowledge that Hamlet now has the lethal foil.
Laertes is already feeling misgivings about his underhanded plan to kill Hamlet by trickery. He says:
(aside to King) My lord, I'll hit him now.
And then to himself:
(aside) And yet 'tis almost 'gainst my conscience.
When Hamlet wounds him with the poisonous foil, Laertes feels he is getting just what he deserves. By telling Osric that he is like a woodcock to his own springe (or snare), i.e., caught in his own trap, he shows his regret and his change of heart, which prepares him for confessing his treachery, advising Hamlet that
The treacherous instrument is in thy hand,
Unbated and envenomed.
and warning him:
The King, the King's to blame.
Thus finally leading to the assassination of Claudius, which has been the dominant objective since Hamlet's first encournter with his father's ghost in Act 1.
Laertes recognises that he has fallen victim to his own treachery, caught in the trap (springe') that he set for Hamlet through the poisoning of the sword. He also sees it as just reward for the 'treachery' that he has been a part of - it leads to his suggestion that he and Hamlet should exchange forgivenesses at the end of the play.