Certain organic compounds can absorb shorter wave-length ultra-violet light and re-emit it at longer wave-lengths in visible spectrum. Therefore a surface containing a fluorescent compound can emit more than the total amount of daylight that falls on it, giving an intensely brilliant white. Compounds that possess these properties are called Optical Brightening Agents or OBA's. The effect is only operative when the incident light has a significant proportion of ultraviolet rays such as sunlight. When OBA treated fabric is exposed to UV, it glows in the dark.
OBA is not a substitute for bleaching. It is used to obtain brilliant market whites. These "white" whites can be obtained without over bleaching and damaging the fibre. On cellulose, it has poor wash fastness but most commercial laundry detergents contain OBA's so they are constantly replenished.