This quote addresses the idea that young men generally believe that "love" is what they are feeling when they see a beautiful woman. Remember, Romeo and Juliet were young adolescents who were mostly driven by hormones and misconcieved longings. Romeo, as a young man, has just proclaimed his love for Juliet one day after he was pining for Rosaline. The friar is questioning Romeo's ability to identify true love since he has fallen immediately in love with Juliet upon seeing her and forgotten Rosaline in that same instant. Love, as Romeo believes he is in, and to which the friar is referring, should not be so quickly changed from one girl to another.
This quote further supports that Romeo is a fickle creature. He professes his love over and over--all his friends tease him about his many girlfriends and how he is never truly serious about any of them. So, looking back at the quote, Romeo's "love" is not truly in his heart but only in his eyes...what he sees.
Friar Lawrence makes this statement in Act II, Scene III after Romeo tells him:
" . . . my heart's dear love is set / On the fair daughter of rich Capulet. / As mine on hers, so hers is set on mine, / And all combin'd, save what thou must combine / By holy marriage."
Friar Lawrence expresses surprise because just the day before, Romeo had professed his love for Rosaline. Today, he wants the friar to marry him to Juliet. The friar calls Romeo's judgment into question and suggests that Romeo is being fickle and making a decision based on appearance and attraction rather than true love.