Interference of light refers to the interaction between coherent waves (waves from same source or of similar frequencies) that are travelling in the same medium.
There are two types of interference, constructive and destructive.
If the crest or trough of one wave meets the crest or trough of another wave at the same point and frequency, i.e the wave is in phase magnitude of their displacement is the sum of individual magnitudes; this is called constructive interference. The waves can be seen as getting amplified. Once the waves pass each other, they return to their normal amplitude.
Destructive interference is like constructive interference except in this the waves are seen as cancelling themselves. When the waves are out of phase, i.e. when the crest of one wave coincides with the trough of another, the resulting amplitude would be the difference of the two waves.
In the case of a CD, first the light gets refracted by the layer of polycarbonate, dispersing light into 7 colours due to change in surrounding medium. The light then gets reflected by the aluminium. The bumps/pits on the aluminium sheet reflect the light in various different directions causing constructive and destructive interference. This is the reason why we see different bands of different colour when we change our angle of observation
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