The tension in a guitar string will affect the speed of the wave traveling through the string as it vibrates. This affects the frequency and changes the pitch of the sound played by the instrument.
Suppose you place your finger on a guitar string, restricting its length to 52 cm. You pluck the string and feel your finger vibrating as the standing wave oscillates back and forth. (Assume the mass density of the guitar strings is 36 g/m).