Interference is the addition of two or more waves that results in a new wave pattern. Interference usually refers to the interaction of waves that are correlated or coherent with each other either because they come from the same source or because they have the same or nearly the same frequency.
Waves don't usually reflect when they strike other waves. Instead, they combine. If the amplitudes of two waves have the same sign (either both positive or both negative), they will add together to form a wave with a larger amplitude. This is called constructive interference. If the two amplitudes have opposite signs, they will subtract to form a combined wave with a lower amplitude. This is called destructive interference.
Sound waves with higher amplitudes sound louder than sound waves with lower amplitudes. Constructive interference will make a sound louder while destructive interference will make a sound quieter.
Two waves that add together may have different frequencies. That means that the crests and troughs won't add up the same way with each new wave because one is moving faster than the other. Part of the waves will interfere constructively and part will interfere destructively.