As with any question that asks for an analysis of one given quote, your first stop must be to look at this quote in context. This quote is the final line of Act I scene 3, a scene in which Rosalind is banished from court because she is the daughter of Duke Senior by Celia's father, Duke Ferdinand. Having received this news, Celia and Rosalind plan to leave court and go to the Forest of Arden together to seek Rosalind's father, with Rosalind disguised as a man and taking Touchstone the Fool with them. Having decided on a plan of what to do, Celia's final lines reflect the way that the power of the mind can transform adversity. Instead of feeling depressed and abused, they are both now able to look upon their banishment as a journey towards liberty rather than the shocking piece of news that it initially was.
Thus we can argue that the quote reflects the mind's ability to transform bad news into good news, overcoming negative feelings and emotions and looking upon life with optimistic eyes.