A hydrophone array is made up of a number of hydrophones placed in known locations. These hydrophones maybe placed in a line on the seafloor, moored in a vertical line in the water column, or towed in a horizontal line behind a boat or ship, for example. Sound arriving at the array from a distant source, such as a submarine, will reach each hydrophone at slightly different times, depending on the direction from which the sound is coming. This time difference is known as the time-of-arrival-difference and can be turned into a direction. Using this information from all the hydrophones in the array, the direction from which the sound is coming can be pinpointed.
Even a simple array consisting of only two hydrophones can give the approximate direction from which a sound is coming. People do this all the time in air with a "receiving array" that consists of two ears. Sound arriving from a source, such as a person speaking, will reach each ear at slightly different times, depending on the direction from which the sound is coming, making it possible for the listener to tell the direction to the speaker.
When the listener wants to detect a single specific sound, hydrophone arrays are much better than single hydrophones. This is because the array is able to filter out noise coming in from all directions and focus on sounds arriving from a specific direction. The increased signal-to-noise ratio allows sounds that normally couldn't be detected by a single hydrophone to be heard. If a hydrophone array is being used to receive a specific sound source, it also allows the source to be quieter and still be detected. For example, the projector in an underwater communication system can be quieter if the receiver is an array pointed at the source.
Hydrophone arrays are used to locate submarines, track marine mammals, and even to study global climate change by detecting temperature differences. Examples of hydrophone arrays include the Sound Surveillance System (SOSUS), which is a set of fixed hydrophone arrays on the seafloor, and the US Navy towed array known as the Surveillance Towed Array Sensor System (SURTASS).