Instantized flour is a low-protein, pre-gelatinized 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. Two well-known commercial brands are "Wondra" and "Shake & Blend".
If instantized flour is not available, all-purpose or cake flour can be substituted but the effect will not be identical. If the flour is used to thicken a sauce, but you may have to take added precautions to avoid lumps in the sauce (i.e., stirrimg like mad). 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.