What warning did that Laertes give Ophelia in Hamlet?

Laertes tells Ophelia not to have a relationship with Hamlet.

Laertes, like many brothers, worries about who has his sister’s heart.  He does not approve of Ophelia having a relationship with Hamlet.

For Hamlet and the trifling of his favour,
Hold it a fashion and a toy in blood,
A violet in the youth of primy nature,
Forward, not permanent, sweet, not lasting,
The perfume and suppliance of a minute; No more. (Act 1, Scene 3)

As far as Laertes is concerned, Hamlet is just playing with his sister.  He does not believe that the prince’s affections for her are true.  Essentially, he thinks that Hamlet is being a player, abusing his position as prince when he really has no intentions for her.

Perhaps he loves you now,
And now no soil nor cautel doth besmirch
The virtue of his will: but you must fear,
His greatness weigh'd, his will is not his own;
For he himself is subject to his birth:
He may not, as unvalued persons do … (Act 1, Scene 3)

Laertes fears that Hamlet will impinge on Ophelia’s honor.  He knows that Hamlet is intelligent but volatile. 

Hamlet uses the situation with Ophelia to hide the real reason for his depression.  As he acts strangely, everyone assumes that it is about Ophelia.  Unfortunately, as Hamlet gets crazier and crazier, Ophelia’s mental and emotional state also suffers.  Hamlet may be acting, but Ophelia is not.

Hamlet's spurning of Ophelia, and the fact that he killed her father, leads Ophelia to suicide.  Hamlet has serious things on his mind with his father's murder and the remarriage of his mother to his uncle, and he does not fully realize what his actions are doing to Ophelia.  When he spurns her, she does more than cry into her pillow at night.  She slowly descends into depressive madness.