In Act 1, Scene 3 of Shakespeare's Hamlet, Polonius shows that he has a highly suspicious nature and is very protective of both his son Laertes and his daughter Ophelia. He questions Ophelia about her relationship with Prince Hamlet and receives her replies with great apparent skepticism. At one point she tells him:
My lord, he hath importuned me with love
In honorable fashion.
. . . . . . . . . . . . . . .
And hath given countenance to his speech
With almost all the holy vows of heaven.
It is "the holy vows of heaven" that Polonius dismisses with the metaphor "springes to catch woodcocks." My edition of the play footnotes this line as "snares, traps to catch (1) foolish birds, (2) fools."
I do know,
When the blood burns, how prodigal the soul
Lends the tongue vows.
And he concludes his lengthy admonition to his daughter with unusually plain words for a man who is used to speaking in metaphors and circumlocutions:
I would not, in plain terms, from this time forth,
Have you to slander any moment leisure,
As to give words or talk with the Lord Hamlet.
Oddly enough, Polonius does not tell Claudius or Gertrude that he considers Hamlet's professions of love "springes to catch woodcocks" or "blazes, having more light than heat, extinct in both." Instead, he tells the King and Queen in Act 2, Scene 2 that Hamlet is madly in love with his daughter. Polonius would, of course, like very much to believe that this is the case, because it could lead to Ophelia becoming queen and placing himself in a very desirable position.
Evidently Polonius' warning to his daughter is not entirely sincere. He probably believes Hamlet is in love with Ophelia, or at least half in love with her, and thinks it would be good policy for his daughter to play hard-to-get.
A springe is a snare or trap, and a woodcock is traditionally thought of as a pretty dumb bird. So, Polonius is basically berating Ophelia by saying that Hamlet's professions of love are just fabrications intended to fool her into thinking he loves her -- and she is as dumb as a woodcock that she would believe it.