Explanation

When two identical waves move in opposite directions, they give rise to stationary waves. 

Beats:

Beats occur when two waves with slightly different frequencies interfere, producing a fluctuating amplitude that can be heard as a pulsating sound. 

Constructive interference:

This happens when two waves with the same phase overlap, leading to an increased amplitude. 

Destructive interference:

This happens when two waves with opposite phases overlap, leading to a decreased or zero amplitude.