Constructive Interference: happens when a crest from one source meets a crest from another source, the energy combines to displace the medium. This also occurs when a trough meets a trough
Destructive Interference: when a crest and a trough meet, the energies combine to work against each other, they tend to cancel out.
Noise Canceling Headphones
They use destructive interference to cancel out the noise coming from the outside as well the headphones use specific material and padding that cancel the noise even further.