Some people don’t like when it’s too quiet. For some it’s uneasy, for some it makes it too hard to stop your mind from wandering. That can make it hard to sleep or hard to focus on a task. White noise is noise that makes it not quiet, but isn’t so loud or too full of meaning to be a distraction on its own. Most people use something like ocean waves, running water, rain, wind noises, the warp core noise from Star Trek, etc.

https://en.wikipedia.org/wiki/White_noise#Work_environment

Just background noise to listen to, because sometimes you just don't like silence.

White noise sounds like radio static.

https://mynoise.net/NoiseMachines/whiteNoiseGenerator.php

People listen to it for many reasons.

There are many kind of noise. But white noise has all frequencies of sound at about the same volume. (Just like white light is a mix of many colors in equal portions.) Think of an old TV on a blank channel, a rainstorm, or a waterfall.