Sound images refer to the visual representations of radar pulses, usually digital, to present a picture of what is happening in places we cannot see.
One of the most commonly understood examples of sound imaging is ultrasound technology, used to "see" inside the human body. Ultrasound imagery is used for a variety of medical applications, but is often associated with pregnancy. Ultrasound imagery allows physicians to "see" a fetus in the womb to determine important things like due dates, size, sex, and possible birth defects.
Similarly, sonar applications allow us to see where we would not otherwise be able to see—for example, in deep ocean environments. Through sonar technologies, we are able to send a sound pulse toward the earth and record its bounce-back rate to determine depth. The application of sonar along the ocean floor can create profiles of ridges and valleys to offer us a better understanding of what is happening deep underwater.
For more information on sonar technology, check out the NOAA Ocean Explorer link, which offers several great pictures produced by sonar technology.