Orlando is asking Oliver if it is possible that he could have fallen so instantly in love with her (we later learn “her” to be Aliena aka Celia). Oliver assures him that it is, and, in light of his engagement with Aliena (who he thinks is a shepherdess), bestows upon Orlando their father’s estate and revenue. Orlando gives his consent to Oliver’s wedding.
Though Oliver had already begun to undergo his conversion, love has hastened and strengthened his moral transformation, and has inspired him to give to Orlando his fair share (and more) of their father’s inheritance. Though it is unclear why Oliver is doing this, it may be because he thinks that Aliena (i.e. Celia) is a shepherdess and so may be giving up his "courtly" life in order to be with her.
Rosalind enters just as Oliver departs, and discusses with Orlando the sudden love between her cousin and his brother. Orlando says he is glad to see his brother’s happiness, but admits that he does not feel happy himself, because his mental imagination of Ganymede as Rosalind can no longer satisfy his longing for the real thing.
Again, a scene potent with dramatic irony and humor: Orlando claims that Ganymede-pretending-to-be-Rosalind can no longer satisfy his longing for the real Rosalind, though of course Ganymede is, in fact, the very person for whom he longs.
Rosalind responds that that she is skilled in the art of magic and promises that, if Orlando loves Rosalind as much as he claims to, he will be married to her tomorrow at the same time when Oliver marries “Aliena.”
Rosalind invokes the power of magic, when actually all she needs to do to bring Rosalind to Orlando is remove her costume.
Silvius and Phebe enter, and Phebe says that she's upset that "Ganymede" shared her letter. Rosalind again tries to persuade Phebe to love her faithful shepherd, Silvius. Silvius then defines love as “all made of sighs and tears…of faith and service… all adoration, duty, and observance…”, and proclaims himself to be all of those things for Phebe, just as Orlando is for Rosalind, Phebe for Ganymede, and Rosalind for no woman.
All the lovers of the play—Silvius, Orlando, Phebe—unite under the common cause of overwhelming love.
Rosalind resolves the scene by telling everyone what will happen the next day: Orlando, Silvius, and she (i.e. Ganymede) will all be married; and she (i.e Ganymede) will satisfy Orlando, content Silvius, and marry Phebe if she ever marries a woman. Silvius, Phebe, and Orlando promise not to fail to show up the next day at Oliver's wedding.
Rosalind again assumes the position of an authority figure on love. While before she was attempting to convince one person or other in or out of love, here she is arranging relationships and setting up marriages to provide resolution to the gender confusion created by her disguise.