If you play every frequency of sound audible to the human ear at the same time and at the same amplitude it will produce white noise.

White noise gets its name from white light, which is a combination of all the different wavelengths of visible light. A prism taken in white light and breaks it into its different components based on its wavelength, thereby producing red light, green light, etc.

Similarly, white noise is a combination of all the different audible wavelengths of sound combined into one. Because white noise contains all frequencies, it is frequently used to mask other sounds. As an example,think of this:

When you're sitting in a hotel room and are having trouble sleeping because the people next door are talking, you turn on the fan to "drown out" the noise. What you are essentially doing is producing white noise with the fan. When you are listening in on two people talking, your brain can usually tune in on one of the people and understand what they are saying. The same is true for three and probably four people. But if you are listening to a room full of 1,000 people talking, your brain has trouble picking out one conversation. The reason being is that a room full of 1,000 people talking sounds a lot like white noise. So, when you turn on that fan in your hotel room you are essentially creating a source of 1,000 voices. Therefore,adding on the one next door to make it 1,001 doesn't really sound any different.