Rosalind tells Orlando that she can cure his madness – that is, she can cure his love – by counseling him (at line 363). She instructs Orlando to imagine that she is the woman he adores (he still thinks she is a man named Ganymede) and then she will act in an inconstant and moody manner like a woman: she will laugh one minute, then cry the next. She will tell him she loves him one instant, then say she hates him the next. In other words, she will so frustrate him that he will never want to love again.
Of course, Rosalind actually has a hidden agenda: she wants to test the sincerity of Orlando’s love.
Although Orlando does not believe that his love can be cured in such a fashion, he agrees to accept the counseling of Ganymede.