Concrete is a very hard, dense surface that sound bounces...

Concrete is a very hard, dense surface that sound bounces off, almost like a billiard ball would bounce off a hard edge. Or if you think of a sound wave as a wave of water, it hits a wall and splashes back as an echo. Or if you think of a flashlight pointed at a mirror, that light would bounce right off and create a glare -- an echo is like an acoustical glare.