An optical brightener is an additive designed to enhance the appearance of colours in fabrics and on papers. It may also enhance or modify the appearance of white items.
The molecules that make up these additives are fluorescent, absorbing energy in the UV portion of the spectrum. This energy is then re-emitted in the visible portion of the spectrum, which modifies the light reflected from the item, which we register as colour. This also means that a white surface treated with an optical brightener may actually reflect more visible spectrum light than shines on it, making it appear brighter.
The additives are commonly used in washing powders, and in paper manufacture, with the result that white paper and white clothing show up as strongly fluorescent under UV illumination. Paper used for banknotes does not contain optical brighteners, so a common method for detecting forged notes is to check for fluorescence.