Vcc- Connect to 5V of positive voltage for power
Trig- A pulse is sent from it for ranging more for object detection
Echo-check the object has been detected or not. If the signal comes back means the object has been detected, if not object has not been detected.
GND- It means Ground terminal, It completes the pathway of the power.

ULTRASONIC SENSOR

This ultrasonic distance sensor provides steady and exact measurements of the distance between 2 cm and 450 cm. It has less than 15 degrees of focus and a range of almost 2 mm.

Ultrasonic sound waves have an extraordinarily high pitch that can not be detected by humans

This particular sensor transmits an ultrasonic sound that is around 40 kHz in frequency. The sensor has two main components-a transducers that generate an ultrasonic sound wave, while its echo is listened to by the other portion

Ultrasonic Distance Measurement Principle :
(https://hackthedeveloper.com/ultrasonic-sensor-hc-sr04-with-arduino-complete-guide/)