What you reap, eventually, is so different from what you sow, and that's an illustration of resurrection. Now in these verses there were two things you notice.
God gives us lessons in nature. He has built into our world a natural process which is taking place all the time, but one which is so marvellous, it almost seems miraculous. That process is a picture of what he will do in the resurrection. It is given us for our encouragement. The seed is sown and if you were to dig it up after some time, you would see that the seed is now a dead husk. The goodness in it has been used up to produce the new plant. In the same way, our bodies are placed in the ground at the end of life, and they truly decay. Of course, nothing grows out of them literally, physically, but the equivalence to the seed is a spiritual equivalence. God will raise up the body again and, to those who have faith in Christ, he gives a glorified resurrection body.