Understand plants with desired characteristics can be developed by selective breeding.
Lets take the rice crop as an example:
The number of rice grains is under the control of genes and if the farmer wants to improve the number of rice grains per plant; he notices that some plants have 6 grains per stem while others may have 8 grains per stem or even 10 grains per stem. So the farmer's decides to harvest the 6 grain and 8 grain plant but he will use the 10 grain ones for planting/breeding.
In the next generation of plants, he noticed that the rice plants are now producing 8 grains per stem, 10 grains per stem or 12 grains per stem. So he would harvest the 8 grain and 10 grain plants and select the 12 grain ones for breeding.
In this way the grain of the rice plant gradually increases which means the yield has increased.