What is white noise?

White noise is a type of noise that is produced by combining sounds of all different frequencies together. If you took all of the imaginable tones that a human can hear and combined them together, you would have white noise.
The adjective "white" is used to describe this type of noise because of the way white light works. White light is light that is made up of all of the different colors (frequencies) of light combined together. In the same way, white noise is a combination of all of the different frequencies of sound. You can think of white noise as 20,000 tones all playing at the same time. Because white noise contains all frequencies, it is frequently used to mask other sounds.