It's a vintage electronic component from before the invention of transistors. Previously they were used in many applications, from radio sets to the first computers. Today their only application is guitar amps where the noisy analog signal they provide makes the sound. Audiophiles claim they can hear the difference between manufacturers of tubes of the same specifications.

As production of vacuum tubes has mostly ceased there is a lot of fetishism for vintage tubes and unused old tubes are sold for significant sums.