This is the scene when Feste the Clown is sent by Olivia to summon Cesario to his mistress' house. Of course, he finds Sebastian, the identical twin of Cesario, and therefore tries to deliver his message to Sebastian. The speech this question refers to runs as follows:
Well held out, i' faith! No, I do not know you; nor
I am not sent to you by my lady, to bid you come
speak with her; nor your name is not Master Cesario;
nor this is not my nose neither. Nothing that is so is so.
The line that ends the speech, "Nothing that is so is so," is actually a very clever ironic reflection on what is happening in the play as a whole, for with the confusion of identity and the mistakes that are complicating the situation, literally everything that appears to be real is not actually real. Feste expresses his annoyance and his frustration with what is happening, as he sees a man who is presumably Cesario deny any knowledge of him. Of course, another way of reading this is to see that Feste is actually expressing the truth: after all, he does not know Sebastian and Sebastian's name is not Cesario.