Light falls on the objects around us. These objects reflect back some of the light. Our eyes detect this light, and our minds form an image. Fluorescence is nothing more than a variation on the very same simple theme. When light falls on objects, the light that is reflected is almost always modified.
White light strikes a blue ball.
The ball absorbs all colors but blue, which it reflects to our eyes. Quite frequently, objects are reflecting more than meets the eye. Because it can tell us things about the object that we cannot otherwise know. Things such as the object's precise chemical composition. Or the presence of foreign substances on or in the object.