White noise is a random signal whose power is spread evenly across the whole frequency range, named by analogy with white light, which mixes all colours. To the ear it sounds like bright static, similar to an untuned radio. Apps and sound machines use it to mask traffic, voices and other intermittent sounds.
The evidence is thinner than its popularity suggests. A systematic review of continuous noise and sleep, published in 2020, rated the evidence that it improves sleep as very low quality and warned that it could also disturb sleep or harm hearing. For attention, a 2024 meta-analysis found a small benefit of white or pink noise on laboratory tasks in children and young adults with ADHD, but a small negative effect in people without ADHD.
Anyone using noise at night does well to keep it quiet and away from the pillow. Persistent sleep or concentration problems are best discussed with a doctor.