In the beginning of the play, Romeo is lovesick over Rosaline. Romeo ends up reading the Capulet guest list for the party and Romeo's friends including Mercutio and Benvolio decide to go. Benvolio encourages Romeo to look at other girls at the party and to forget Rosaline.
Benvolio tells Romeo to follow his lead. Look at other ladies. There are other people than this Rosalind girl. Benvolio hopes that if Romeo sees that there are other girls that may return his affections that will forget Rosalind.
This is also an example of dramatic irony. Romeo has no clue what will happen when takes Benvolio's advice but we as the audience knows that he will meet Juliet soon.