Ok I know this. I learned it in 3rd grade. Its whenever sound waves bounce of an object and then come back to the original point it left. For example, bats use sonar which uses the echo to its advantage. It makes a noise that bounces off of something. If it comes back loud, it's food, if it's quite, it's a rock or something in it's way. It's so high pitched that only the bat can hear it, though.
This may help you research a little further: Light waves are sine waves. Sound waves are compressional waves. If you shout and hear an "echo" it means that the wave is being reflected back to your ears (or a listening device) by an object. As I recall, in order for you to recognize the reflected sound as a "echo," the minimum distance the reflecting object must be from the sound of your voice is 55 feet in .