Polonius is sending Reynaldo to Paris to deliver some money and "notes" to Laertes. This shows that Laertes has been gone from Denmark for some time, since he said goodbye to his father and his sister Ophelia in Act I, Scene 3. The crafty Polonius is concerned about his son's behavior, or possible misbehavior, in Paris. Laertes is young and has never been away from home before. Polonius knows there are many ways for young men to get in trouble when they are on their own. He tells Reynaldo:
You shall do marvelous wisely, good Reynaldo,
Before you visit him, to make inquire
Of his behaviour.
Obviously, Polonius is mainly concerned about keeping some sort of watch over his beloved son. He gives Reynaldo extensive advice about how to "inquire of his behaviour." Evidently he thinks Reynaldo should make such inquiries "before" he visits him so that Laertes won't know Reynaldo is in Paris. Also, this will give Reynaldo an excuse for asking questions about Laertes, as if he were having trouble finding him in the big French city.
Take you, as 'twere, some distant knowledge of him,
As thus, ‘I know his father and his friends,
And in part him.’ Do you mark this, Reynaldo?
‘And in part him, but,’ you may say, ‘not well....
if't be he I mean, he's very wild,
Addicted so and so’; and there put on him
What forgeries you please—marry, none so rank
As may dishonour him, take heed of that—
But, sir, such wanton, wild and usual slips
As are companions noted and most known
To youth and liberty.
Shakespeare introduced Laertes to his audience in Act I, along with all the other principal characters in the play; but the playwright had no present use for the Laertes and sent him off to France until he would be needed much later after the death of his father Polonius. Shakespeare's main purpose for staging the meeting between Polonius and Reynaldo In Act II, Scene 1 seems to be to establish that Laertes is now in far-away Paris and that some time has passed since he left. Laertes had to be introduced in Act I so that the audience would recognize him when he came storming back to Denmark in Act IV, scene 5.