What is an interference?
An interference is a wave that runs into another wave.
Waves can come in many forms. There are waves in water, earthquakes create waves, sound is generated by waves, and to some light is a wave as electromagnetic radiation. When two or more waves converge into each other, they tend to cause some strange effects. The major effects of interference could be either of the three: a constructive interference that combines the two waves, a destructive interference that obliterates the waves, or a type of interference that is not completely constructive nor destructive; it is more of an in-between interference.
This Wikipedia diagram describes constructive and destructive interference respectively. The bottom half depicts the two waves converging while the top half is the end result. Constructive doubles the size of the wave, destructive cancels the waves out.
If any of those bottom waves were offset to the side in any way, they wouldn't be purely constructive or destructive. It would result in an in-between.
A simplified way to determine the interference it to measure the amplitudes (wave height) and add the amplitudes of the two waves at a given point. If one wave has an amplitude of 1 and another also has 1, the end result would make the amplitude 2. Note that waves can travel at different rates, so if there is a wave that travels slower than another, the interference won't be uniformly constructive or destructive any you can end up with an in-between or mixed interference.