Overlap, also called interference in some cases, is the phenomenon that occurs when two or more waves meet, generating a resulting wave equal to the algebraic sum of the perturbations of each wave.
Imagine a horizontally stretched rope producing pulses of the same width, but of varying amplitude, at the ends of the rope, overlapping can happen in two ways:
Situation 1: the pulses are given in phase.
At the moment the pulses meet, their elongations at each point of the rope add algebraically, and their amplitude (maximum elongation) is the sum of the two amplitudes:
After this meeting, each one goes in its initial direction, with its initial characteristics preserved.
This type of overlap is called constructive interference, since overlap causes the amplitude to be momentarily increased in modulus.