If you have two waves, let's assume of the same frequency, you can 'define' a phase difference between them. The phase difference simply tells you how much one wave is 'ahead of' the other wave, and is often given in degrees (0 to 360) or radians (0 to 2pi).
If you look at the following image:
Clearly, the waves have the same frequency, but one wave is slightly ahead of the other: there is a phase difference between the two.In general, there are a few phase differences of special interest: a phase difference of 0 radians (or 0 degrees) means the waves are exactly "in phase", one is not ahead of the other. A phase difference of pi (or 180 degrees) is often called exactly "out of phase", one wave is exactly half a wavelength ahead of the other wave, which means that the peaks of one wave fall together with the troughs ('dips') of the other wave.
When the two waves are for example sound waves (it is equally valid for all other kind of waves) and when they exist in the same place, then the waves will interfere. A phase difference of 0 will then cause the interference wave to be the sum of both waves. With a phase difference of pi (180 degrees), the peaks and troughs will cancel each other out and the resulting wave is zero everywhere: there is no longer a wave.
