Emerson is trying to point out the challenges with remaining as a non- conformist. At the time in which he is writing, America began to embrace a model of industrialization and the notion of generating material wealth became a driving force in society. This helped to create a conformist view of the world, where individuals sought the same ends in the same settings through the same goals. Individuals who spoke out against this were seen as "wrong" or "defective" or "abnormal." They were seen as "different," in a bad way. The reality is that there is a price to be paid for non- conformity because it forces individuals to take a stand against what is being established and understood as "the singular notion of the good." Emerson is pointing out how much of a challenge this is and while he is not in favor of it, he is trying to suggest that this is what reality is and individuals should understand this in order to change it. To some extent, this is how things are today. While we are doing better with accepting the idea of difference, there are some social settings where conformity is an absolute. In these domains, Emerson's words of both condition and change ring true.
Emerson means that people punish the nonconformist but it doesn't matter because the self-reliant person should stand up against public opinion. He says:
For nonconformity the world whips you with its displeasure. And therefore a man must know how to estimate a sour face. The by-standers look askance on him in the public street or in the friend’s parlour. If this aversation had its origin in contempt and resistance like his own, he might well go home with a sad countenance; but the sour faces of the multitude, like their sweet faces, have no deep cause, but are put on and off as the wind blows and a newspaper directs.
He is saying that the people's sour faces or sweet faces have no real meaning because they come and go like the wind. It is so true. Public opinion is fickle or arbitrary.
Emerson would say it is better to be true to yourself and forget what people think or how they react.