In this scene, Polonius is curious to know whether Laertes is engaged in any shady business or wrongdoings so he sends his servant Reynaldo to find out. However, Polonius betrays his son as he asks Reynaldo to find out what Laertes is doing by spreading false rumors about him in town. The method Polonius uses to spy on his son is dishonest because Polonius deliberately wants to tell bad rumors about his son to find out the truth based on the reactions of others. This incident helps the audience understand that betrayal can even come from those close to you and that it results from impure intentions like those of Polonius.
Gee I wonder how my son Laertes is doing...I need to check up on him and make sure he isn't doing anything shady or impure...
I know! I'll get Reynaldo to spy on him!
Listen Reynaldo, go find out what Laertes is doing BUT first ask about him around the town. Get an idea of what they think of him and you know what? You can even tell lies about him! But don't dishonor him or anything! Make sense?
Dishonor him but don't dishonor him...right...
I don't know if you get what I'm trying to do here but if you lie and spread rumors about my son, you can get people to tell the truth about how he actually is. That way you can spy on him and I can know if he's doing indecent and dishonest things. I'm a genius.
Jeez didn't know fathers would want to backstab their sons