This means that this is an instrument used to map and find things usually in the ocean. Sonar devices use sound waves to find things. The sound waves bounce off objects. If a sound bounces of the object and returns quickly, the object is close by. If the sound does not come back quickly, the object must be deep in the ocean.
Some governments use sonar for radar to find submarines and military equipment.
Active sonar used a projector to send out a sound wave that spreads outward and is bounced back by an object. Passive sonar picks up sensors that pick up sound from an object that makes a noise such as whales, submarines and ships. Sometimes sonar buoys are placed on the ocean floor to listen in whenever the researchers want to hear what is going on at the bottom. Acoustic sonar need a projector and receiver at both ends. These devices let ships communicate with other ships.