Interference is a phenomenon of superposition of 2 waves to form a resultant wave of greater, lower or the same amplitude.
When the crest of a wave overlaps the crest of another wave of the same frequency at the same point, then the resultant amplitude will be the sum of the amplitudes of individual waves. Then it is known as constructive interference.
Amplitude of the resultant wave will be more as compared to the amplitude of individual waves.
When the crest of one wave meets the trough of another wave,then the resultant amplitude is given as difference of the two individual amplitudes.Then it is known as destructive interference.
Amplitude of the resultant wave will be less as compared to the amplitude of individual waves.
If two stones are dropped in the river,ripples or wavesare generated by both stones.When both the waves overlap,there will be maximum displacement when the waves are in phase and there will be no displacement when they are out of phase.