What is a sheaf of wheat?

Answer 1

Wheat is a form of grass. Like grass, it grows in individual stalks with the actual edible wheat at the top of the stalk. Each stalk is tall. You cut it off at the bottom. It is very hard to carry a bunch of stalks if they are loose so the farmer ties a whole lot of them together, making a big bundle. Each bundle can then be tossed onto a vehicle and taken to the place where the important parts are removed for human use.

That tied-together bundle is the "sheaf".