There are 2 types of interference:
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Constructive is when 2 waves with the same sign on their amplitude merge to create a larger wave. Think about when you 'construct' a building, you build it up just like the waves are going up.
Destructive is when 2 waves with opposite signs on their amplitude merge to reduce their amplitude. Think about destruction, you brake things down to smaller pieces. The wave here is being made smaller.