This is known as constructive interference , and the waves are said to be in phase with each other at that position and time. Destructive interference takes place at positions and times at which interfering waves are out of phase with each other. That is, they have displacements of opposite sign and tend to cancel each other out. Beats from two tuning forks. Interference effects are most commonly and easily observed with waves from coherent sources.
Coherent waves have a fixed relationship in time.
Imagine two speakers at different positions that oscillate identically at a fixed frequency. At any given position in space, the relative phase of waves is fixed. Hence, there are fixed positions at which the waves from the two sources interfere constructively, leading to large oscillations. There are also fixed positions at which the waves interfere destructively, leading to small oscillations. The behavior of the waves at a particular position and time depends only on the behavior of the sources when they were produced and the distances they have traveled.