When an undulatory movement meets an obstacle in its propagation, the wavefronts change direction, they are reflected.  

To study this phenomenon, we will imagine that a wavefront is moving across a pond towards the edge formed by a vertical wall. If this wall does not absorb the energy transmitted by the wave, where will it go after the collision?

In the visual we have provided to study reflection, you can vary the direction of the wavefront by dragging the end of the direction arrow.