Instant flour is a low-protein, pregelatinized wheat flour to which some malted barley flour has been added. It has been formulated to dissolve quickly in either hot or cold liquids, and is most often called for to thicken gravies and sauces.
Because of its low-protein content, it is also sometimes used in making pie crusts and other recipes that call for cake flour, which is also lower in protein than all-purpose flour. Some bakers find the slightly acidic taste of cake flour objectionable and prefer the instant flour. The leading brand available in this country is Wondra.
If instant flour is hard to find where you live, you can substitute all-purpose or cake flour. But you want to recognize what the recipe writer is trying to accomplish in calling for instant flour. If the flour is used to thicken a sauce, you can certainly use all-purpose flour, but you may have to take added precautions to avoid the development of lumps in the sauce (i.e., stir like mad, use an immersion blender). If the instant flour is used in a baking project and is called for in a relatively large quantity, it is probably specified because of its protein content, and your best option would be cake or pastry flour.