Answer:
There are two kinds of sonar: One is passive sonar, and the other is active sonar. For passive sonar, we don't send sound waves but just receive them. By listening to these special sound waves, animals that communicate with these particular sound waves can be detected. By listening to the sound waves from engines of submarines or ships, we can locate them.
Also, other things that vibrates can be detected by passive sonar. Many threats on the sea are usually detected by passive sonar this way. Since passive sonar does not transmit any thing to the environment, it does no harm to other animals. Active sonar is a little different.
Besides having a receiver, active sonar also transmits sound waves. Active sonar works by transmiting particular sound pulse and measuring the length of time for the sound to be reflected back, then devide the time by 2 to get the time it takes for the sound pulse to reach the object