Answer: You get interference when two wave motions meet.

You get reinforcement (constructive interference) or destruction (destructive interference) of two waves when they get 'instep' or 'out of step'. When the waves are 'in step' (known as 'in phase') they strengthen each other. When the waves are 'out of step' (known as 'out of phase') they weaken each other.

The diagram shows the two waves are in phase, crest matching crest and trough matching trough at the points labelled R. Here the two waves reinforce each other. If the waves have other frequencies at other places, marked D, they must be out of phase, with a crest meeting a trough. At these places, the waves weaken or destroy each other. This interaction between waves is called interference.

The interference happens in all kind of waves. The other enclosed picture (originally one of our photographs, which does not scan well) shows a pattern of water waves and the interference pattern.

A good way to see wave patterns is to use a ripple tank, which your school or college should be able to demonstrate to you.