Emerson's essay argues passionately that a person must trust his or her inner voice: that we all have a destiny given to us by God, and we all have to find it and cultivate it for ourselves. This is what Emerson means by self-reliance. We can't borrow the knowledge of who God meant us to be by conforming to other people's ideas. Emerson, among many other images, tries to impress on us that we can't really be nourished properly trying to eat someone else's corn. That's not a helpful shortcut, because eating what someone else has planted, nurtured and grown will leave us dissatisfied. We have to do our own internal work.
For Biblically literate audiences of Emerson's time, Jesus's words you "reap what you sow" would have leaped to mind with this image. The entire essay leans heavily, if in a more universal (transcendental) manner, into the Protestant idea of examination of conscience. Don't rely on an earthly authority or society to tell you what is right for you to do: that is an issue between your conscience and your God. Once you get your vocation or purpose right, everything else will fall into place. Your destiny is unique to you. As Emerson says after the "kernel" quote:
The power which resides in him is new in nature, and none but he knows what that is which he can do, nor does he know until he has tried. Not for nothing one face, one character, one fact, makes much impression on him, and another none. This sculpture in the memory is not without preestablished harmony. The eye was placed where one ray should fall, that it might testify of that particular ray. We but half express ourselves, and are ashamed of that divine idea which each of us represents.
He will then follow with a famous line: "Trust thyself: every heart vibrates to that iron string."
Let us consider the full quote that relates to this question and what Emerson is trying to argue through this analogy:
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.
Emerson is making several points that relate to a central argument. We must accept ourselves as humans, and also recognise that there is little point in envying what others have and are given. The only fruits we are going to see in our lives are going to be from our own hard work. The analogy of the corn is used to emphasise this through a comparison to farming and the way that we can only expect good crops on our land if we work hard ourselves on that land to help our crops grow well. In the same way, in our lives, if we want them to be fruitful, we must realise that we will need to work hard ourselves and invest in our lives and what we are doing for our lives to be fruitful.