Here's a breakdown:

* Reflection: The phenomenon where a wave encounters a boundary and changes direction, bouncing back into the original medium.

* Barrier: The surface or object that the wave encounters and reflects off of.

This applies to various types of waves, including:

* Light waves: When light bounces off a mirror.

* Sound waves: When sound waves bounce off a wall, creating an echo.

* Water waves: When waves in a pond hit a barrier like a rock.