Polonius has directed his servant Reynaldo to go to France to discover information about Laertes. Instead of finding Laertes and asking him directly how he is doing, how he is spending his money, how he is passing his free time, with whom is he friends, etc., Reynaldo is told to plant some false rumors among the Danes living in France who know Laertes. Polonius tells Reynaldo, "Your bait of falsehood takes this carp of truth; [...] By indirections by directions out." Using a fishing image, the old man tells his servant to use lies as bait to find out the truth about Laertes. This indirect approach is typical of Polonius, who tries to use deception and spying to discover information about others, most notably Hamlet.
Before this point in the play, however, Polonius has not seemed duplicitous. He advised Laertes before his son left for France, "To thine own self be true," and he seemed genuinely concerned about the relationship between Ophelia and Hamlet as a possible cause for Hamlet's strange behavior. Now Polonius shows his true colors: he is a deceptive, sneaky old man who apparently enjoys spying on people.