Very low-protein wheat flour is used in making it (about 2% protein), along with a small amount of malted barley flour. To manufacture it, the flour is first extrusion-cooked by mechanically treating it with high heat at high pressure, then suddenly returned to normal pressure, at which point the water in the product spontaneously vaporises. Then it is dried, and ground to a granular consistency like that of salt.
Some people also use it as a cake or pastry flour. Some people like using it to make batters with, though many people have had it turn their batters grainy. Some people like to use it for dredging and coating.
Most though would consider instant flour far too expensive to be using for something that plain old flour would do far better. But a canister of it in the cupboard is handy for rescuing gravies and sauces at the last minute.
Do not sift before measuring.
In a sauce, you can substitute any wheat flour, though be prepared to whisk to get out lumps, and to need to cook after adding to get rid of the raw taste. Or, try Beurre Manié, or start off with a Roux.
If called for in a cake recipe, they probably are after something like Cake and Pastry flour, but like most of us you can just use all-purpose or plain flour.