We can say that there are two things that Friar Laurence cautions Romeo of in Act 2, Scene 3. The first caution is with respect to Romeo's youth and fickleness. When Romeo begs Friar Laurence not to "chide," or "scold" him for changing so fast from loving Rosaline to loving Juliet, Romeo argues that his switch is practical because Juliet returns his affection whereas Rosaline did not, as we see in his lines:
I pray thee chide not. She whom I love now
Doth grace for grace and love for love allow.
The other did not so. (87-89)
When Romeo declares that Rosaline did not return his love, Friar Laurence makes the very amusing, yet very true statement that Rosaline did very well not to return Romeo's love, that "she knew better," because Romeo was too young and did not yet actually know what real love was. We see Friar Laurence judging Romeo's youth and knowledge of love in his lines, "O, she knew well / Thy love did read by rote, and could not spell" (90-91). In saying that Romeo's love "reads by rote," he is saying that his love reads by "memory" or by "habit," like a child ("Rote," Random House Dictionary). Children, before they actually learn to read, will often pick up their favorite books that have been read to them often enough that they have memorized every word and pretend to read the words. Of course the children are not actually reading, they are just reciting what they have memorized. Therefore, in saying that Romeo's "love" reads by memory, he is calling Romeo a child who reads by memory rather than true, learned, adult skill. Romeo may have learned from adults around him what love feels like, but he does not yet truly know what love is. Hence, the first thing that Friar Laurence is cautioning Romeo against is his idea that he truly loves Juliet, or truly loves at all. He is cautioning Romeo to be more wise and realize that he does not yet know what love is.
The second thing Friar Laurence cautions Romeo about can be seen in his final line in this scene. Friar Laurence says, "Wisely, and slow. They stumble that run fast" (97). In saying this, Friar Laurence is advising Romeo that he is being too hasty in his decision to marry Juliet. He is warning him that being hasty only brings all sorts of grief.