It is the physical effect of adding individual wave characteristics into a new, combined waveform.
The principle of superposition is sometimes stated as follows:
When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location. http://www.physicsclassroom.com/class/waves/Lesson-3/Interference-of-Waves
A nice animation is here: http://www.acs.psu.edu/drussell/Demos/superposition/superposition.html