In this essay, Emerson encourages each person to live by their own principles. He exhorts the readers to rely on themselves for their ideas and values and goals, rather than listening to what other people say they should think, value, or strive for. The person who is self-reliant is one who is, ultimately, more mature and happy and satisfied with life than one who fails to listen to themselves and to learn what makes them unique and worthy.
Thus, Emerson says,
There is a time in every man’s education when he arrives at the conviction that envy is ignorance; that imitation is suicide; that he must take himself for better, for worse, as his portion; that though the wide universe is full of good, no kernel of nourishing corn can come to him but through his toil bestowed on that plot of ground which is given to him to till.
In other words, this realization only comes once one has reached a certain level of maturity: when we envy another person, we remain in ignorance of ourselves and what we have to offer; further, when we imitate another person, perhaps out of envy, we kill the independent and unique part of ourselves. This is because we are trying to be something we are not.
Each of us must accept who we are—the good and the bad—because we can do nothing real and true and lasting until we work with what gifts each of us is given. We must, to use Emerson's metaphor, till our own plot of ground, developing it and seeing what will grow there. When we try to plant what someone else has planted, so to speak, we do a disservice to our plot. Often, we might be better suited to growing something different, perhaps even something new.