Laertes is off to university in Paris. Upon his departure, his father, Polonius, gives him a list of dos and don'ts, one of which—“Neither a borrower nor a lender be”—has become a staple of the English language.
That Polonius should take to lecturing his son like this on how to conduct himself should surprise no one. Polonius is a pompous finger-wagger, never happier than when he's telling other people what to do.
Even though Laertes comes across as quite a sensible, level-headed young man, his father still feels the need to lay down the law to him, an indication that Polonius, as well as his many other faults, is also something of a control-freak.
As a loyal and faithful son, it's highly unlikely in the extreme that Laertes will get up to any mischief while he's in Paris. But Polonius is not prepared to take any chances. He sends his servant Reynaldo after Laertes to make sure that his son stays out of trouble.
Once he's arrived in Paris, Reynaldo is to start spreading rumors about Laertes's dissolute behavior and see how people react. If they react with incredulity, as if the very idea that Laertes could do such things is simply too ridiculous for words, then Reynaldo will know that Polonius's son is behaving himself.