An underbase is a layer of ink (generally white or other light color) that is printed as a "base" on a dark fabric for other colors to sit on. This gives the top colors more brilliance. Since the underbase is generally a high opacity ink, it is flash-cured before the top colors are printed over it. Overall, printing an underlay significantly slows down production due to two additional steps (base print + flash curing). Underbases are also called an underlay (very common) or a mask (not very common).
Although you could print each color as a high opacity ink and flash after each color, an underbase allows the print to have a bright look yet be soft to the touch because the underbase is printed through lower mesh counts and the top colors very high mesh counts.