This is part of Polonius' parting advice to his son Laertes as he is leaving for France. In Act I, Scene 3, Polonius advises his son,
"Costly thy habit as thy purse can buy,
But not express'd in fancy; rich, not gaudy; (75)
For the apparel oft proclaims the man,
Polonius is telling his son to buy
". . . as costly clothes as can pay for,
But not made fancy, rich, and certainly not gaudy.
For the clothes often tell what kind of man you are,"
In other words, we are often judged by our appearance and since the French are very cautious about their appearance, Laertes should also consider his clothing. They should not necessarily be the most expensive, but should as expensive as he can afford.