grain

Definition:

The term "grain" usually refers to the seed of a food plant such as wheat, barley, corn, millet, or rice. It can also refer to the whole plant.

In the Bible, the main grains that are referred to are wheat and barley.
The heads of grain are the part of the plant that holds the grain.
Note that some older Bible versions use the word "corn" to refer to grain in general. In modern English however, "corn" only refers to one type of grain.