A magecraft that uses magical energy to materialize a mirror image of an object.
As Rin mentioned in the story, it's not very efficient.
That's because, if you're making a replica of something, rather than using projection it's easier and more practical just to gather the materials and build it yourself.
Normally, it is only used to project a substitute for an item that has already been lost, and for just a few minutes within one's own time axis.
At any rate, since Gradation Air is just supposed to be a short-term rental of the item's outward appearance, you can see why the version used by Shirou (and Archer) is completely ridiculous. It's no wonder that Rin wanted to kill him when she saw his shed.
Of course, Shirou can only project things that are related to weapons in some way—or more specifically, swords. Modern weapons are off limits.
…Well, technically he can pull out shields or armor, too, if he strains himself to his utmost limits, but the effects only last for an instant and the cost is enormous.