Easy thing. When an opaque material will come closer to the sensor, the buzzer will make sound LED will light up.

You can watch the video - https://www.youtube.com/watch?v=zklsAf4Zpoc 


Upon searching the web, I I found that there is no proper complete guide of this basic thing. In this guide, we will use Regular IR Obstacle Sensor, which cheap; not Sharp IR Sensor. You’ll see that our diagram shown is of Sharp IR Sensor, but actually connection is same and for Regular IR Obstacle Sensor.