When two groups of waves meet and overlap they interfere with each other.
The resulting amplitude will depend on the amplitudes of both the waves at that point.

If the crest of one wave meets the crest of the other the waves are said to be in phase and the resulting intensity will be large.
This is constructive interference.

If the crest of one wave meets the trough of the other (and the waves are of equal amplitude) they are out of phase by p then the resulting intensity will be zero. This is destructive interference.
This phase difference may be produced by allowing the two sets of waves to travel different distances - this difference in distance of travel is called the path difference between the two waves.