"Envy is ignorance" because it prevents the development of an individual. To wish to be like someone else is to deny one's own potential--"suicide"--and, thus, to remain in the darkness of ignorance. Known for his repeated phrase of "trust thyself," Emerson repudiated the idea of a man's wishing to conform in his envy.
Emerson's essay, Self-Reliance, begins with what he felt was genius:
To believe your own thought, to believe what is true for you in your private heart is true for all men--that is genius.
Trusting oneself, and being an individual are genius, the antithesis of envy. Emerson contends that God has made each person unique with his/her own individual work to do. Therefore, to trust one's own thoughts and actions is very much like listening to God and acting upon His wishes.
"Society everywhere is in conspiracy against the manhood of every one of its members."
For Emerson, the importance of the individual is paramount. "Self-reliance is its [conformity's] aversion." The integrity of one's mind is what matters, not imitation, not envy.
In order to understand this line, let us look at a bit more of the essay -- "your" line and a part that comes after it. Emerson says that a man has to know
that envy is ignorance; that imitation is suicide; that he must take himself for better, for worse, as his portion
What Emerson is saying here is one of the major ideas of transcendentalism -- he is saying that you have to be yourself. Transcendentalists like Emerson belive that people have to obey their own consciences rather than listening to what society tells them to do. In this quote, Emerson is saying much the same thing.
He is saying that it is dumb to want to be like other people (to envy them). He is saying that trying to imitate other people is like killing yourself. He is saying that you have to be content with who you are.
So, the major idea here is that you have to be yourself -- you cannot be what anyone else is or what anyone else thinks you should be.
The idea of paying mind to the subjective notion of self is a powerfully compelling idea that is featured in Emerson's work. The notion of envy is the desire to focus the subjective on another individual and within this, Emerson is suggesting that one loses their most elemental quality. The idea of the subjective voice being the only element that an individual truly owns or possesses is an idea that is very powerful to the Transcendentalists. These thinkers saw society as a growing cosmopolitan mass where individuals were defining themselves more in competition with one another, and not in distinction with one another. It is not surprising that the rise of industry was happening as the Transcendentalists were composing their ideas. Emerson's ideas of being distinctive, heeding the call of the individual, and praising the subjective as an experience with its own intrinsic good are evident in his belief that envy and imitation detract from these notions.
Along with the idea of being true to yourself, I would argue that Emerson is also implying something about the nature of envy itself. We generally are envious of other people, often people that we don't know very much about.
The idea of enying something or someone implies a desire to be in that position but what Emerson makes clear is that one of the main reasons we envy that person or that position is our ignorance. If we really understood or knew everything about that position, we might no longer want it.
Particularly going along with the idea of being true to one's own ideals and feelings, who knows what we might have to give up to have that thing or to be with that person, etc. The idea is that if we really knew everything about it, we'd no longer be envious.
The main theme of Ralph Waldo Emerson's essay "Self Reliance" is that every human being is a unique individual and that he or she must do his or her best to promote his or her own unique qualities. No matter what happens he or she must not imitate others for that would be suicidal according to Emerson as it would result in one losing his or her unique nature. So one must not envy others as it only reveals his or her ignorance of his or her own special unique nature. On the contrary one must work hard to make manifest one's unique power and nature without envying others: