Bright or dark bands caused by how much two waves overlap each other. If the waves are ‘in phase’ then the two waves are lined up perfectly. If the waves are ‘out of phase’ then the waves are perfectly misaligned.
If we consider light, two waves of the same brightness which are in phase would create a point twice as bright as the original waves. Two waves of the same brightness which are out of phase with one another would create a point which is completely dark.