Sound travels in waves, much the same way ripples of water travel in the ocean. When you make a sound, sound waves travel through the air. You first hear that sound when the sound waves reach your ears.
However, if those sound waves hit an obstacle like a building or the walls of a cave, they bounce back into the air because those objects do not absorb sound. Instead, they reflect, or bounce the sound back, and you hear it a second time. That second sound is called an echo.
If only one obstacle reflects the sound, you’ll hear only one echo, a simple echo. But if many surfaces reflect the echo, bouncing it from surface to surface, like in a valley or canyon surrounded by mountains, you hear the echo over and over, but it grows fainter at each repetition. This repeated echo is called a compound echo, or reverberation.
Heavy, low-hanging clouds can also reflect sounds and create echoes.
Echoes can actually measure how far you are standing from the echo-producing surface, since sound waves travel at about one mile every five seconds!