What is a constructive interference?
Constructive interference occurs when the waves are in phase, and destructive interference when they are half a cycle out of phase.
If we add these two waves together, point-by-point, we end up with a new wave that looks pretty much like the original waves but its amplitude is larger. This situation, where the resultant wave is bigger than either of the two original, is called constructive interference. The waves are adding together to form a bigger wave.