Before talking about light, lets consider water waves. Image two water waves having different wavelengths and traveling towards one another. These two waves will interact, and produce a surface with very different characteristics. This interaction of the two waves is called interference . Constructive interference occurs when the crest of one wave is aligned with the crest of the other and the two waves produce large crests and deep troughs.
Constructive interference occurs when the waves are in-phase.
In the case of light, constructive interference produces bright bands, and destructive interference dark bands. Interference occurs with all waves. If you are a musician, or not a musician, you may have noticed that when two different notes are played a regular series of pulses is heard. An advantage of the wave theory of light is that it explains how light can bend around objects. Though the waves cannot penetrate the pier, at the end of the pier the waves bend around this obstacle.