When the crest of one wave passes through, or is superpositioned upon, the crest of another wave, we say that the waves constructively interfere. Constructive interference also occurs when the trough of one wave is superpositioned upon the trough of another wave.
During any wave interference the shape of the medium is determined by the sum of the separate amplitudes of each wave. We often say that when waves interfere, amplitudes add.
Watch the video above. Notice that in this animation the crest of the red wave constructively interferes with the crest of the blue wave.