Superposition

The principle of superposition applies to when more than two waves are travelling through the same medium at the same time.The waves are undisturbed but the displacement is changed to the sum of the waves.
There are two types, constructive and destructive. For example, when two waves are coming at each other, constructive is when the waves are on the same side of the axis, so that when they pass over each other, they will build more in their direction.
 
While for destructive interference, the waves are on the opposite side of each other and one will subtract the other and the resulting wave will be smaller.