ADS-B OUT and ADS-B IN are two fundamental concepts in ADS-B technology. The first thing we need to understand is that both ADS-B OUT and ADS-B IN refer to specific functions that aircraft can have.
Just like how a person speaks to communicate their whereabouts and intentions, ADS-B OUT can indeed be likened to the "mouth" of the aircraft, constantly “speaking” (broadcasting) its own vital information to other aircrafts and ground stations. ADS-B OUT function allow the aircraft to “be seen”.
Just like how a person uses his/her ears to listen and gather information from his/her surroundings, ADS-B IN functions as the "ears" of the aircraft, continuously “listening to” (receiving) the ADS-B information broadcasted by nearby aircrafts. ADS-B IN function allows the aircraft to “see”.
For example, Aircraft A, equipped with ADS-B OUT, continuously broadcasts its own ADS-B information. Aircraft B, equipped with ADS-B IN, receives the ADS-B information transmitted by aircraft A. This allows aircraft B to "see" or detect aircraft A's presence and know its position, speed, and other details.
Well, we may say one more word. For ground-to-air surveillance, where air traffic control (ATC) needs to monitor aircrafts from the ground, ADS-B OUT alone is sufficient. On the other hand, for air-to-air surveillance applications, where aircrafts need to detect and avoid each other in the sky, both ADS-B OUT and ADS-B IN are necessary.