Sublimation is a technique using special ink and paper to print your design, that once pressed or at very high heat actually becomes embedded into the fibers of the substrate.  It does not work well on natural fibers, ie: cotton, it is mainly used on man-made fibers or items coated with a special chemical that allows the sublimation ink to penetrate the item.  Sublimation substrates are white to allow for the colors in the design to be vibrant.