When you pluck the string of a guitar, the vibration which you produce travels to both ends of the string where it is reflected. This phenomenon is repeated many times, so that what we can perceive  in the string is the superposition of two identical undulatory movements, travelling in opposite directions.

The result of this superposition is called a stationary wave because it seems to produce a wave which is "frozen" in space and does not propagate in any direction.

In phenomena like light and sound it is difficult to perceive this circumstance because it is a problem to create two identical beams of light or sound moving in opposite directions. With regard to sound you can read in an encyclopaedia how Kundt managed to do this with a tube filled with sand.

We will study the case of the waves which cross on a guitar string.