Optoreflector sensors contain a matched infrared transmitter (LED) and infrared receiver (usually a phototransistor) pair. These devices work by measuring the amount of light that is reflected into the receiver. Because the receiver also responds to ambient light, the device works best when well shielded from abient light, and when the distance between the sensor and the reflective surface is small (the graph below shows how distance affects the output value). IR reflectance sensors are often used to detect white and black surfaces. White surfaces generally reflect well, while black surfaces reflect poorly.
Warning: If you make the sensor too sensitive, ambient light may be able to trigger it.