An elementary magecraft, but one that’s considered virtually impossible to master.
Though passing magical energy through an object to enhance its existence is the basis of all magecraft, perhaps because of the high degree of freedom and lack of clear executable forms, there are not very many almighty “strengthening” specialists.
Naturally, Shirou is a dropout who is only capable of succeeding at strengthening that involves weapons.
Since what’s being enhanced is the meaning of an object’s existence, using it on a knife will increase its sharpness, using it on food will increase its nutrition, and using it on a maid will increase her cuteness.
Of course, if an object’s purpose is too vague, it will be impossible to strengthen it.
Also, because it’s hard to pass your own magical energy through a living being, the “strengthening” of other people is considered to be a feat of the greatest difficulty.

Fate/side material: Fate Encyclopedia