What does it mean when a string on your guitar tighten?

When a string on your guitar tightens, the tension in the string increases. This causes the string to vibrate at a higher frequency, which in turn produces a higher-pitched sound. It can also cause the string to stretch and become thinner, which can also affect the sound. Guitar strings tighten gradually over time due to normal use, but they can also tighten suddenly if they are exposed to excessive heat or humidity.