I assume that you are talking about the advice Benvolio gives to Romeo in Act I, Scene 1. In that scene, we see Romeo for the first time. He is moping around because he is lovesick. He is totally in love with this woman named Rosaline but she is not really interested in him.
Benvolio's advice to Romeo is to just quit thinking about Rosaline. Romeo cannot believe this is possible and asks Benvolio how he is supposed to do that. Benvolio tells him to do it by looking at other girls. He figures that Romeo should look at other beautiful girls and that will make him forget Rosaline.
In general terms, Benvolio advises Romeo to cure his obsession with the unattainable Rosaline by looking at other beautiful ladies. But if you look at the text closely, you'll see he does a bit more than this.
First, he hints that any passionate love on Romeo's part is probably undesirable. If he should fall in love with another lady, that would amount to an affliction—just like his love for Rosaline. The implication is that Romeo would be better off if he didn't get so swept up in romantic feelings.
Second, he doesn't just tell Romeo to go out looking. In Act I, Scene ii, Benvolio refines his idea to include a live, side-by-side comparison of Rosaline with someone else. He suggests that Romeo only thinks Rosaline is very beautiful because he saw her in isolation. When he sees her in the same room with the competition, he will revise his views.
As noted by others who have answered this question, Benvolio's initial advice occurs in Act I, Scene i. He tells Romeo to stop thinking about Rosaline. Then, when Romeo asks how, Benvolio answers that Romeo should turn his attention towards other beautiful ladies:
By giving liberty unto thine eyes;
Examine other beauties
But if we skip ahead to the second scene of Act I we find more. There Benvolio reiterates his theme, but now he presents it using metaphors about fire, pain, and disease:
Tut, man, one fire burns out another's burning,
One pain is lessen'd by another's anguish;
Turn giddy, and be holp by backward turning;
One desperate grief cures with another's languish:
Take thou some new infection to thy eye,
And the rank poison of the old will die.
Next, Benvolio seizes on the opportunity of the party. Rosaline will be there, and so will "all the admired beauties of Verona." This is where Benvolio tells Romeo to make those direct comparisons. He assures Romeo that Rosaline will seem unattractive when they are done:
Go thither; and, with unattainted eye,
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.
Finally, Benvolio suggests that Romeo has overrated Rosaline's appearance because he viewed her in isolation ("Tut, you saw her fair, none else being by"). We may interpret this as additional advice from Benvolio: Don't fixate on someone until you've actively compared her with some alternatives.
I think you are referring to Benvolio and Romeo's discussion about Rosaline. Romeo shares that he is in love with a woman who doesn't want to love him back, her excuse is that she plans to live chaste. This occurs in Act I, scene i.
Benvolio suggests first that he forget Rosaline. The he suggests that Romeo check out other women.
Here's specifically what he said:
By giving liberty unto thine eyes;
Examine other beauties.
They take advantage of this opportunity in Act I, scene v by going to a party at the Capulet's house.