In lines 86-89 of Act 1, Scene 2, Benvolio is trying to persuade Romeo to forget about Rosaline and to consider other women instead. When Capulet's servant comes down the street with the list of guests he is supposed to invite to the Capulet's ball but cannot because he can't read, Romeo reads the list for the servant, and both Romeo and Benvolio learn that Rosaline, as a relation of Capulet's, will be attending the ball. This news gives Bevolio the idea that if only Romeo can see Rosaline in comparison to other beautiful women, Romeo would forget all about Rosaline. Hence, when Benvolio speaks lines 86-89, he is first pointing out the fact that Rosaline will be there dining with the other beautiful women of Verona, as we see in his lines:
At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lov'st;
With all the admired beauties of Verona. (86-88)
Next, Benvolio begs Romeo to go to the ball and judge all the beautiful faces without prejudice, as we see in the line, "Go thither, and with unattainted eye...," with "unattainted" being translated as "unprejudiced" (89; eNotes). Finally, he says that if Romeo can compare Rosaline to other beauties, he will soon see that other women are really far more beautiful and that Rosaline is really ordinary compared to other women, as we see in his analogy, "Compare her face with some that I shall show, / And I will make thee think thy swan a crow" (90-91). Since swans are very beautiful birds and much rarer than ordinary birds like crows, and since crows are very common, black, dull, and ugly, he is using the analogy of comparing a swan to a crow to show that Rosaline is really plain and common compared to other beautiful women.