Polonius wants Reynaldo to ask around about what Laertes has been doing before he sees him. He tells him to ask leading questions and drop statements about Laertes' bad habits ('he's very wild') to try and get people to tell him what Laertes has really been up to. Polonius says that if they know Laertes is really doing all those things they'll be comfortable with Reynaldo and that's how he'll know what Laertes has really been doing.
This shows that Polonius is not very trusting and is very willing to use deception wherever he can in order to get the information that he wants.
