Gelatinization occurs when granules of a starch product, such as flour or cornstarch, swell when they come in contact with a liquid. This swelling process causes the the starch granules to thicken the liquid in which they are immersed. This is the basis for many gravies and sauces used in cooking.
In nearly all cases, gelatinization occurs when heat is applied to the liquid and starch mixture, usually in a range of between 130-185 degrees Fahrenheit. A form of gelatinization also occurs when starch products like rice and pasta are cooked. The result is thicker, softer grains of rice and soft, pliable pasta noodles.