It is designed for line sensing applications and comprised of two parts - an IR emitting LED and an IR sensitive phototransistor. It detects white and black color, and it outputs via TTL signal, so the robot can reliably follow a black line on a white background, or vice versa. You can even combine several sensors to meet your specific requirements.
Marking "D" represents the module is a digital sensor,the other side graph represents function of the sensor. Easy identification!