If you're wondering where to store your grain, the simple answer is — in a granary. Not much more to say, really. A granary is where grain is stored.
One clear way to remember the meaning of granary is that it sounds like what it is. If I were to ask you to invent a word that means "the place where grain is stored" you'd very likely come up with granary — though admittedly, the spelling is a little different. In the same way that wine comes from a winery, you'll find granaries the world over filled with, well, grain.