Answer: In echolocation, you use wave sounds to found objects:
A source emanates a wave of sound, and the sound travels in the medium, eventually rebounds on a wall or something, and the wave returns to the source. Then the source is listening the echoes of the sound ( this is the motive of the name)
Now, if some object is between the walls and the source, then when the wave reaches that object, a small part of the wave will rebound first than the rest of it, and the thing using echolocation will know that there is an object and the exact position of the object.
This is something that some animals use to move in dark places, for example the bats use echolocation when they do that little noises in caves.
