IntroIf you're starting with Arduino, you won't get past this sensor. It's a cheap, easy-to-use ultrasonic distance sensor. If you ever wanted to build your own obstacle avoiding robot, you'll need at least one of these sensors.
How it worksIn simple words. There is a speaker that emits an ultrasonic impulse. The pulse is not audible for the human ear. The pulse travels with the speed of sound. And when it hits a target, it's reflected back to the sensor, where a microphone can detect it. And because we know how fast sound travels, we can calculate by the duration of the impulse to came back, the distance to the object.
I attached the trigger pin to Digital 2 (D2) and the echo pin to Digital 3 (D3). But you can use any digital pins.Although I wouldn't recommend using Pin 0 or Pin 1, cos' it would probably interfere with the serial monitor.
Watch the video. It probably will answer all your questions.