Explain what happens waves undergo constructive interference

The principle of superposition states that when 2 waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point.

Based on the above definition, two waves constructively interfere when they are in phase.

In another words, if a crest of one wave meets a crest of another, the combined amplitude will be the sum of the amplitudes (constructive interference). Yet if a crest meets a trough, you get destructive

The following includes some diagrams which make this a lot easier to understand:

https://en.wikipedia.org/wiki/Interference_(wave_propagation)