Two waves with the same frequency and travelling in opposite directions interfere with one another and form a standing wave. The standing wave is the lower (black) waveform.

Move the time slider and the two waves traveling in opposite directions (red and blue waves) can be observed in the upper portion.[br]Those waves are added and form the standing wave displayed in the lower portion of the animation (black).[br]Observe that there are parts of the standing wave that do not oscillate, these are the "nodes", the nodes are separated in space by half a wavelength of the constituent waves.