An underbase is a layer of ink that goes underneath a print to cover a dark or colored shirt. Inks are somewhat transparent, so printing on a bright red shirt will tint all of the ink red, and printing on a shirt that is darker than the ink will make it appear darker. To solve this, we print a layer of white ink, then cure it so it doesn't mix with the other inks, then the other colors are printed on top of the white base without being affected by the fabric color.