• Ophelia instructs her brother,

Do not, as some ungracious pastors do,
Show me the steep and thorny way to heaven, 
Whilst, like a puff'd and reckless libertine, 
Himself the primrose path of dalliance treads 
And recks not his own rede. (1.3.50-54)

• There is, also, a positive significance to the speech of Laertes. For, in contrast to the corrupt, "incestuous," "villainous," and alienated family of Hamlet, there is a familial bond between brother and sister and father in the family of Polonius.
• In addition, with his words about Hamlet's being a prince, Laertes suggests by saying a new form of government for Denmark. For, Denmark did not have a system in which sons inherited the throne and power; rather, a Council of nobles chose the king. So, in his suggestion, Laertes places himself outside the Danish court, forming a triangle with Hamlet and Fortinbras.

William Shakespeare's Hamlet is primarily the story of Hamlet and his quest for revenge against Claudius; however, in the midst of his conflicts, it is clear that Hamlet has a relationship of some sort with Ophelia. This relationship eventually costs Ophelia her life, and it is a relationship that her brother warns her about, though perhaps not for the correct reasons.

Before he returns to school, Laertes and Ophelia have a few moments together. It is clear that they love each other and have an open and honest relationship, even though Ophelia later scolds her brother, teasingly, about preaching to her but not following his own advice about things. In this speech, Laertes warns Ophelia about several specific worries he has concerning her relationship with Hamlet, something she readily admits to having. 

His first warning is that Hamlet, as the Prince of Denmark, does not have the right to select his own wife. He tells 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,
Carve for himself; for on his choice depends
The safety and health of this whole state;
And therefore must his choice be circumscribed
Unto the voice and yielding of that body
Whereof he is the head.

Laertes compares Hamlet to a body (metaphor) and calls Denmark the voice and body to which Hamlet, as the head, must yield. Laertes is afraid Ophelia will be hurt when Hamlet has to choose his wife based on the political expedience rather than love. 

Laertes also warns his sister that Hamlet will tell her anything she wants to hear, but she should not listen too closely to his protestations of love and guard her chastity. She will suffer the consequences if she gives herself to Hamlet outside of marriage. 

Then weigh what loss your honour may sustain,
If with too credent ear you list his songs,
Or lose your heart, or your chaste treasure open
To his unmaster'd importunity.

Laertes does not want Ophelia to "lose her heart" (personification, for a heart cannot actually be lost) or open her "chaste treasure" (a metaphor for losing her valuable and precious gift of virginity) just because Hamlet begs her for them. Even the lowest maid, he tells her, is sinful "if she "unmask her beauty to the moon" (spoils her virtue). She must not give in to temptation, especially since she is "in the morn and liquid dew of youth" and more susceptible to such temptations. 

Laertes's final warning is for Ophelia to start living in fear of the dangers which might occur if she continues to engage in a relationship with Hamlet:

Be wary then; best safety lies in fear:
Youth to itself rebels, though none else near.

Unfortunately, even though Ophelia obeys both her brother and her father, she suffers from having a relationship with Hamlet, a suffering that eventually leads her to take her own life. 

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.