When sound waves strike a surface, they reflect off of that surface and can return to the source of the sound as an echo. To a listener, this may be identical to the original sound, just delayed and possibly distorted by its path through the air. If the echo arrives quickly enough, it may seem to be part of the original sound, forming a reverberation instead of an echo.
When something makes a sound in air, sound waves travel in all directions from the source. These waves propagate through the air, compressing the air molecules into waves. Any time these waves strike a surface, they reflect away from it, and in a large, empty area, the waves can bounce around, creating multiple echoes.
In room-temperature air, sound travels at approximately 343 meters per second. This means that if the nearest wall is more than 17 meters away, the sound may take more than a tenth of a second to return, and creates a distinct echo instead of a simple reverberation effect. Echoes off walls closer than 17 meters arrive before the sound fades, creating a distinctive persistent quality in the sound itself.
Echoes are the basis of both sonar and radar systems. Sonar uses sound pulses to measure distances to objects underwater, while radar uses radio waves to accomplish a similar task in air.